Avoid leaking pixel memory if DUMMYScreenInit fails
Resolves issues reported by Oracle Parfait static analyzer:
Error: Memory leak
Memory leak [memory-leak] (CWE 401):
Memory leak of pointer pixels allocated with malloc((pScrn->videoRam * 1024))
at line 803 of dummy_driver.c in function 'DUMMYScreenInit'.
pixels allocated at line 790 with malloc((pScrn->videoRam * 1024))
Memory leak [memory-leak] (CWE 401):
Memory leak of pointer pixels allocated with malloc((pScrn->videoRam * 1024))
at line 805 of dummy_driver.c in function 'DUMMYScreenInit'.
pixels allocated at line 790 with malloc((pScrn->videoRam * 1024))