Avoid setting unused variables (-Wself-assign)

Setting a variable and then assigning it to itself avoids
-Wunused-but-set-variable but this specific trick is now caught by
-Wself-assign. Instead, actually use the value or don't bother
assigning it at all:

gdbusauth.c: call g_data_input_stream_read_byte() in void context
gdbusauthmechanismsha1.c: value is actually used
gdbusmessage.c: use consistent preprocessor-token protection
gthreadedresolver.c: skip over bytes in data blob
httpd.c: do something useful with the value

https://bugzilla.gnome.org/show_bug.cgi?id=745723

gio/gdbusauthmechanismsha1.c

@@ -753,7 +753,6 @@ keyring_generate_entry (const gchar  *cookie_context,
               g_strfreev (tokens);
               goto out;
             }
-          line_when = line_when; /* To avoid -Wunused-but-set-variable */
 
 
           /* D-Bus spec says: