glamor: fixes GL_INVALID_ENUM errors on ES if there is no quads

If there is no quads to draw, then we have a possibility to call
glDrawElements with type as zero, which will generate
GL_INVALID_ENUM error. While this error is harmless, it is annoying.

Signed-off-by: Konstantin <ria.freelander@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

glamor/glamor.c

@@ -307,6 +307,10 @@ glamor_gldrawarrays_quads_using_indices(glamor_screen_private *glamor_priv,
 {
     unsigned i;
 
+    /* If there is no quads to draw, just exit */
+    if (count == 0)
+        return;
+
     /* For a single quad, don't bother with an index buffer. */
     if (count ==  1)
         goto fallback;