Check that error exists before trying to set it. Fixes bug #640975.
Check that error exists before trying to set it. Fixes bug #640975.

djdeath/glib,cention-sany/glib,cosimoc/glib,01org/android-bluez-glib,lukasz-skalski/glib,bratsche/glib,Distrotech/glib,justinkb/aosp-bluez.glib,darren-clark/android_platform_external_bluetooth_glib,ahmedammar/platform_external_gst_glib,mzabaluev/glib,tchakabam/glib,antono/glib,bluez-android/glib,justinkb/aosp-bluez.glib,iConsole/Console-OS_external_bluetooth_glib,ahmedammar/platform_external_gst_glib,endlessm/glib,darren-clark/android_platform_external_bluetooth_glib,djdeath/glib,mzabaluev/glib,djdeath/glib,antono/glib,tchakabam/glib,cosimoc/glib,01org/android-bluez-glib,johne53/MB3Glib,tamaskenez/glib,gale320/glib,Distrotech/glib,tamaskenez/glib,iConsole/Console-OS_external_bluetooth_glib,johne53/MB3Glib,bluez-android/glib,Distrotech/glib,ieei/glib,zsx/glib,tchakabam/glib,lukasz-skalski/glib,gale320/glib,djdeath/glib,bluez-android/glib,pstglia/platform-external-bluetooth-glib,djdeath/glib,krichter722/glib,gale320/glib,krichter722/glib,pstglia/external-bluetooth-glib,krichter722/glib,MathieuDuponchelle/glib,justinkb/aosp-bluez.glib,johne53/MB3Glib,cosimoc/glib,ieei/glib,bratsche/glib,antono/glib,johne53/MB3Glib,ieei/glib,gale320/glib,MathieuDuponchelle/glib,zsx/glib,ieei/glib,cosimoc/glib,cention-sany/glib,lukasz-skalski/glib,endlessm/glib,cention-sany/glib,pstglia/platform-external-bluetooth-glib,lukasz-skalski/glib,darren-clark/android_platform_external_bluetooth_glib,bratsche/glib,ahmedammar/platform_external_gst_glib,endlessm/glib,zsx/glib,darren-clark/android_platform_external_bluetooth_glib,Distrotech/glib,mzabaluev/glib,iConsole/Console-OS_external_bluetooth_glib,pstglia/external-bluetooth-glib,cosimoc/glib,01org/android-bluez-glib,mzabaluev/glib,krichter722/glib,justinkb/aosp-bluez.glib,ahmedammar/platform_external_gst_glib,tchakabam/glib,01org/android-bluez-glib,cention-sany/glib,Distrotech/glib,bratsche/glib,tchakabam/glib,pstglia/platform-external-bluetooth-glib,01org/android-bluez-glib,darren-clark/android_platform_external_bluetooth_glib,MathieuDuponchelle/glib,pstglia/external-bluetooth-glib,MathieuDuponchelle/glib,pstglia/external-bluetooth-glib,pstglia/platform-external-bluetooth-glib,justinkb/aosp-bluez.glib,cention-sany/glib,tamaskenez/glib,bluez-android/glib,gale320/glib,endlessm/glib,pstglia/external-bluetooth-glib,antono/glib,johne53/MB3Glib,pstglia/platform-external-bluetooth-glib,tamaskenez/glib,mzabaluev/glib,bluez-android/glib,johne53/MB3Glib,ieei/glib,endlessm/glib,iConsole/Console-OS_external_bluetooth_glib,lukasz-skalski/glib,tamaskenez/glib,iConsole/Console-OS_external_bluetooth_glib,MathieuDuponchelle/glib,krichter722/glib,zsx/glib