xfree86: Return NULL from xf86CompatOutput if no compat_output is defined

If no compat_output is defined, we inadvertently (attempt to) return
whatever data is at index -1. Instead, return NULL since that's what
callers are expecting.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>

hw/xfree86/modes/xf86Crtc.h

@@ -745,6 +745,8 @@ xf86CompatOutput(ScrnInfoPtr pScrn)
 {
     xf86CrtcConfigPtr config = XF86_CRTC_CONFIG_PTR(pScrn);
 
+    if (config->compat_output < 0)
+        return NULL;
     return config->output[config->compat_output];
 }