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/tests/httpd.c

@@ -67,12 +67,19 @@ handler (GThreadedSocketService *service,
 
   version = NULL;
   tmp = strchr (escaped, ' ');
-  if (tmp != NULL)
+  if (tmp == NULL)
     {
-      *tmp = 0;
-      version = tmp + 1;
+      send_error (out, 400, "Bad Request");
+      goto out;
+    }
+  *tmp = 0;
+
+  version = tmp + 1;
+  if (!g_str_has_prefix (version, "HTTP/1."))
+    {
+      send_error(out, 505, "HTTP Version Not Supported");
+      goto out;
     }
-  version = version; /* To avoid -Wunused-but-set-variable */
 
   query = strchr (escaped, '?');
   if (query != NULL)