NestedClientCreateScreen: avoid leaks on failure paths
Found by Oracle Parfait 13.3 static analyzer:
Memory leak [memory-leak]:
Memory leak of pointer pPriv allocated with malloc(168)
at line 182 of xlibclient.c in function 'NestedClientCreateScreen'.
pPriv allocated at line 177 with malloc(168)
Signed-off-by: Alan Coopersmith