glx: protect createModeFromConfig() from allocation failure

| ../glx/glxdricommon.c: In function ‘createModeFromConfig’:
| ../glx/glxdricommon.c:142:23: warning: dereference of possibly-NULL ‘config’ [CWE-690] [-Wanalyzer-possible-null-dereference]
|   142 |     config->driConfig = driConfig;

Consumers can already handle returning NULL, so this seems the best compromise.
It will look like we don't have any modes at all. Certainly not nice, but at
least better than completely crashing the Xserver.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

glx/glxdricommon.c

@@ -136,6 +136,8 @@ createModeFromConfig(const __DRIcoreExtension * core,
 
 
     config = calloc(1, sizeof *config);
+    if (!config)
+        return NULL;
 
     config->driConfig = driConfig;