fix umm_malloc null ptr. (endless reboot) like: ``` Fatal exception (28): epc1=0x402097e8, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000001, depc=0x00000000 Fatal exception (28): epc1=0x402097e8, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000001, depc=0x00000000 Fatal exception (28): epc1=0x402097e8, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000001, depc=0x00000000 ```
fix umm_malloc null ptr.
(endless reboot) like:
```
Fatal exception (28):
epc1=0x402097e8, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000001, depc=0x00000000
Fatal exception (28):
epc1=0x402097e8, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000001, depc=0x00000000
Fatal exception (28):
epc1=0x402097e8, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000001, depc=0x00000000
```

```
0x402097e8: umm_info at umm_malloc/umm_malloc.c line 1000
```

gguuss/Arduino,KaloNK/Arduino,me-no-dev/Arduino,gguuss/Arduino,KaloNK/Arduino,Adam5Wu/Arduino,esp8266/Arduino,wemos/Arduino,sticilface/Arduino,gguuss/Arduino,Adam5Wu/Arduino,Adam5Wu/Arduino,toastedcode/esp8266-Arduino,KaloNK/Arduino,quertenmont/Arduino,toastedcode/esp8266-Arduino,quertenmont/Arduino,Juppit/Arduino,esp8266/Arduino,hallard/Arduino,jes/Arduino,me-no-dev/Arduino,me-no-dev/Arduino,Juppit/Arduino,wemos/Arduino,jes/Arduino,martinayotte/ESP8266-Arduino,lrmoreno007/Arduino,lrmoreno007/Arduino,martinayotte/ESP8266-Arduino,Juppit/Arduino,hallard/Arduino,Links2004/Arduino,lrmoreno007/Arduino,sticilface/Arduino,hallard/Arduino,NullMedia/Arduino,sticilface/Arduino,KaloNK/Arduino,gguuss/Arduino,me-no-dev/Arduino,hallard/Arduino,Juppit/Arduino,esp8266/Arduino,NullMedia/Arduino,wemos/Arduino,Links2004/Arduino,quertenmont/Arduino,martinayotte/ESP8266-Arduino,martinayotte/ESP8266-Arduino,quertenmont/Arduino,NullMedia/Arduino,jes/Arduino,Links2004/Arduino,jes/Arduino,Links2004/Arduino,Juppit/Arduino,lrmoreno007/Arduino,esp8266/Arduino,esp8266/Arduino,hallard/Arduino,wemos/Arduino,Adam5Wu/Arduino,toastedcode/esp8266-Arduino,KaloNK/Arduino,Lan-Hekary/Arduino,NullMedia/Arduino,Lan-Hekary/Arduino,Lan-Hekary/Arduino,toastedcode/esp8266-Arduino,Adam5Wu/Arduino,sticilface/Arduino,Links2004/Arduino,sticilface/Arduino,wemos/Arduino,Lan-Hekary/Arduino,martinayotte/ESP8266-Arduino,quertenmont/Arduino,NullMedia/Arduino,me-no-dev/Arduino,jes/Arduino,Lan-Hekary/Arduino,toastedcode/esp8266-Arduino,gguuss/Arduino,lrmoreno007/Arduino