dtc: Fix zero-length input segfault
dtc: Fix zero-length input segfault

This patch fixes a segmentation fault caused by dereferencing a NULL
pointer (pos->file aka yylloc.file) in srcpos_string when the input
length is 0 (fe 'dtc </dev/null'.) Reason: yylloc.file is initialized
with 0 and the tokenizer, which updates yylloc.file via srcpos_update
doesn't get a chance to run on zero-length input.

Signed-off-by: Horst Kronstorfer <0e4472ad3ee94e555ad93d950978c5b54abe5910@frequentis.com>

detule/lk-g2-spr,oubeichen/lk-v500,CanarySolutions/little-kernel,CanarySolutions/little-kernel,lg-devs/g2-bootloader,jsr-d10/android_bootable_bootloader_lk,DooMLoRD/android_bootable_bootloader_lk,t2m-foxfone/kernel_lk,pichina/lk,hanjae/lumiab0,t2m-foxfone/kernel_lk,zhuotong/mi2_lk_NoEmmc,MiCode/mi2_lk,efidroid/lk,jsr-d10/android_bootable_bootloader_lk,mozilla-b2g/kernel_lk,oubeichen/lk-v500,M1cha/lktris,hanjae/lumiab0,jsr-d9/android_kernel_lk,MiCode/mi2_lk,DooMLoRD/android_bootable_bootloader_lk,M1cha/lktris,mozilla-b2g/kernel_lk,Redmi-dev/android_bootable_lk,pichina/lk,lg-devs/g2-bootloader,mozilla-b2g/kernel_lk,Blefish/android_bootable_bootloader_lk,hanjae/lumiab0,M1cha/android_bootable_bootloader_lk,efidroid/lk,Foxda-Tech/argo8-bootable-bootloader-lk,idor/dk50-bootable_bootloader_lk,jsr-d9/android_kernel_lk,Redmi-dev/android_bootable_lk,chirayudesai/android_bootable_bootloader_lk,zhuotong/mi2_lk_NoEmmc,thornbirdblue/codeaurora_lk,M1cha/lktris,CanarySolutions/little-kernel,zhuotong/mi2_lk_NoEmmc,oubeichen/lk-v500,DooMLoRD/android_bootable_bootloader_lk,idor/dk50-bootable_bootloader_lk,detule/lk-g2-spr,mozilla-b2g/fairphone2_kernel_lk,hanjae/lumiab0,M1cha/android_bootable_bootloader_lk,Foxda-Tech/polaris-bootable-bootloader-lk,RonGokhale/android_lk_bootloader,Foxda-Tech/argo8-bootable-bootloader-lk,jsr-d10/android_bootable_bootloader_lk,thornbirdblue/codeaurora_lk,chirayudesai/android_bootable_bootloader_lk,M1cha/lktris,mozilla-b2g/kernel_lk,Foxda-Tech/argo8-bootable-bootloader-lk,jsr-d10/android_bootable_bootloader_lk,jbott/lk_gee,M1cha/android_bootable_bootloader_lk,DooMLoRD/android_bootable_bootloader_lk,RonGokhale/android_lk_bootloader,chirayudesai/android_bootable_bootloader_lk,detule/lk-g2-spr,mozilla-b2g/kernel_lk,thornbirdblue/codeaurora_lk,M1cha/lktris,utilite2/lk,jsr-d9/android_kernel_lk,detule/lk-g2-spr,Foxda-Tech/polaris-bootable-bootloader-lk,idor/dk50-bootable_bootloader_lk,Foxda-Tech/polaris-bootable-bootloader-lk,lg-devs/g2-bootloader,RonGokhale/android_lk_bootloader,Blefish/android_bootable_bootloader_lk,t2m-foxfone/android_kernel_lk,thornbirdblue/codeaurora_lk,mozilla-b2g/fairphone2_kernel_lk,t2m-foxfone/kernel_lk,efidroid/lk,pichina/lk,M1cha/mi2_lk,lg-devs/g2-bootloader,utilite2/lk,M1cha/android_bootable_bootloader_lk,utilite2/lk,detule/lk-g2-spr,jsr-d10/android_bootable_bootloader_lk,idor/dk50-bootable_bootloader_lk,jbott/lk_gee,jbott/lk_gee,Redmi-dev/android_bootable_lk,M1cha/mi2_lk,zhuotong/mi2_lk_NoEmmc,Foxda-Tech/argo8-bootable-bootloader-lk,Redmi-dev/android_bootable_lk,jsr-d9/android_kernel_lk,pichina/lk,M1cha/android_bootable_bootloader_lk,oubeichen/lk-v500,Foxda-Tech/polaris-bootable-bootloader-lk,jbott/lk_gee,t2m-foxfone/android_kernel_lk,Foxda-Tech/polaris-bootable-bootloader-lk,chirayudesai/android_bootable_bootloader_lk,M1cha/mi2_lk,hanjae/lumiab0,oubeichen/lk-v500,RonGokhale/android_lk_bootloader,CanarySolutions/little-kernel,mozilla-b2g/fairphone2_kernel_lk,pichina/lk,t2m-foxfone/android_kernel_lk,Blefish/android_bootable_bootloader_lk,chirayudesai/android_bootable_bootloader_lk,efidroid/lk,jsr-d9/android_kernel_lk,mozilla-b2g/fairphone2_kernel_lk,DooMLoRD/android_bootable_bootloader_lk,idor/dk50-bootable_bootloader_lk,M1cha/mi2_lk,thornbirdblue/codeaurora_lk,t2m-foxfone/android_kernel_lk,lg-devs/g2-bootloader,MiCode/mi2_lk,jbott/lk_gee,RonGokhale/android_lk_bootloader,M1cha/mi2_lk,M1cha/android_bootable_bootloader_lk,t2m-foxfone/kernel_lk,MiCode/mi2_lk,CanarySolutions/little-kernel,utilite2/lk,t2m-foxfone/kernel_lk,zhuotong/mi2_lk_NoEmmc,t2m-foxfone/android_kernel_lk,utilite2/lk,Blefish/android_bootable_bootloader_lk,mozilla-b2g/fairphone2_kernel_lk,efidroid/lk,MiCode/mi2_lk,Foxda-Tech/argo8-bootable-bootloader-lk,Blefish/android_bootable_bootloader_lk,Redmi-dev/android_bootable_lk