volume monitor: don't unref NULL
volume monitor: don't unref NULL

Fix a couple more cases of blindly calling g_object_unref() on the
result of a function that is documented as sometimes returning NULL.

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