pidgin/pidgin

Fix a few issues with the CredentialProvider code that snuck past review

Testing Done:
Compiled and ran unit tests locally. Verified the warnings were gone from the unit test build and verified the warning was gone from the Purple-3.0.gir build.

Reviewed at https://reviews.imfreedom.org/r/168/
if PLUGINS
if target_machine.system() != 'windows'
CANBERRA = dependency('libcanberra', version: '>= 0.30')
library('notification-sound', 'notification-sound.c',
dependencies : [libpurple_dep, glib, CANBERRA],
name_prefix : '',
install : true, install_dir : PURPLE_PLUGINDIR)
endif
endif