glX HasExtension(): check for NULL ext string first.

It's slightly faster than failing later, after a strchr() call, since this
will get called multiple times with a NULL string if the system totally
fails elsewhere.

src/video/x11/SDL_x11opengl.c

@@ -279,14 +279,14 @@ HasExtension(const char *extension, const char *extensions)
     const char *start;
     const char *where, *terminator;
 
+    if (!extensions)
+        return SDL_FALSE;
+
     /* Extension names should not have spaces. */
     where = SDL_strchr(extension, ' ');
     if (where || *extension == '\0')
         return SDL_FALSE;
 
-    if (!extensions)
-        return SDL_FALSE;
-
     /* It takes a bit of care to be fool-proof about parsing the
      * OpenGL extensions string. Don't be fooled by sub-strings,
      * etc. */