Decrement mode count when removing RandR output mode.

Removing an output mode without decrementing the mode count scrambles the
output mode array badly.

randr/rroutput.c

@@ -249,6 +249,7 @@ RROutputDeleteUserMode (RROutputPtr output,
 
     memmove (output->userModes + m, output->userModes + m + 1,
 	     (output->numUserModes - m - 1) * sizeof (RRModePtr));
+    output->numUserModes--;
     RRModeDestroy (mode);
     return Success;
 }