video/sis/: fix negative array index

video/sis/: fix negative array index

This patch fixes the possible usage of a negative value as an array
index spotted by the Coverity checker.

sisfb_validate_mode() could return a negative error code and we must check for
that prior to using its return value as an array index.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>