gfile: don't report completion twice on g_file_load_contents error
gfile: don't report completion twice on g_file_load_contents error

When an error occurs while reading the file input stream in
g_file_load_contents (e.g. because the operation was cancelled), the
code is correctly calling g_task_return_error(), but in the callback
from the close operation, g_task_return_boolean() will be called again.

Code that cleans up its state in the async callback will then be called
twice, leading to invalid memory access.

https://bugzilla.gnome.org/show_bug.cgi?id=692202

cosimoc/glib,krichter722/glib,djdeath/glib,krichter722/glib,tchakabam/glib,johne53/MB3Glib,cention-sany/glib,lukasz-skalski/glib,endlessm/glib,djdeath/glib,Distrotech/glib,ieei/glib,cosimoc/glib,ieei/glib,johne53/MB3Glib,lukasz-skalski/glib,tamaskenez/glib,ieei/glib,gale320/glib,mzabaluev/glib,cention-sany/glib,tamaskenez/glib,johne53/MB3Glib,lukasz-skalski/glib,mzabaluev/glib,cention-sany/glib,tamaskenez/glib,gale320/glib,endlessm/glib,gale320/glib,MathieuDuponchelle/glib,gale320/glib,ieei/glib,cosimoc/glib,krichter722/glib,johne53/MB3Glib,mzabaluev/glib,endlessm/glib,djdeath/glib,gale320/glib,endlessm/glib,cosimoc/glib,johne53/MB3Glib,mzabaluev/glib,cention-sany/glib,MathieuDuponchelle/glib,krichter722/glib,krichter722/glib,Distrotech/glib,Distrotech/glib,lukasz-skalski/glib,MathieuDuponchelle/glib,johne53/MB3Glib,tchakabam/glib,ieei/glib,cosimoc/glib,tchakabam/glib,tamaskenez/glib,djdeath/glib,MathieuDuponchelle/glib,tchakabam/glib,mzabaluev/glib,djdeath/glib,endlessm/glib,Distrotech/glib,tchakabam/glib,MathieuDuponchelle/glib,tamaskenez/glib,lukasz-skalski/glib,Distrotech/glib,cention-sany/glib