Check for NULL extensions string

Some X server not supporting any OpenGL feature, glXQueryExtensionsString
will return NULL and causes the function to fail.

Thanks to Emmanuel Stapf (manus@eiffel.com) for the original patch.

This was verified running an application on macOS while the X server was
running on Windows Xming 7.5.0.10

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>

src/dispatch_common.c

@@ -347,6 +347,8 @@ epoxy_conservative_gl_version(void)
 bool
 epoxy_extension_in_string(const char *extension_list, const char *ext)
 {
+    if (!extension_list)
+        return false;
     const char *ptr = extension_list;
     int len = strlen(ext);