Prevent a crash when buffer() returns 0

If buffer() returns 0 then there is no bufferPixels available which will
cause a crash later on when it tries to set that memory. If this function
fails then all we can do is return, a warning will have already been
outputted from buffer() itself indicating why.

Change-Id: I5890b3c34536f7f3d17def0936970c0a694b005a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>