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>

drivers/video/sis/sis_main.c

@@ -1248,7 +1248,6 @@ sisfb_do_set_var(struct fb_var_screeninfo *var, int isactive, struct fb_info *in
 	if(found_mode) {
 		ivideo->sisfb_mode_idx = sisfb_validate_mode(ivideo,
 				ivideo->sisfb_mode_idx, ivideo->currentvbflags);
-		ivideo->mode_no = sisbios_mode[ivideo->sisfb_mode_idx].mode_no[ivideo->mni];
 	} else {
 		ivideo->sisfb_mode_idx = -1;
 	}
@@ -1260,6 +1259,8 @@ sisfb_do_set_var(struct fb_var_screeninfo *var, int isactive, struct fb_info *in
 		return -EINVAL;
 	}
 
+	ivideo->mode_no = sisbios_mode[ivideo->sisfb_mode_idx].mode_no[ivideo->mni];
+
 	if(sisfb_search_refresh_rate(ivideo, ivideo->refresh_rate, ivideo->sisfb_mode_idx) == 0) {
 		ivideo->rate_idx = sisbios_mode[ivideo->sisfb_mode_idx].rate_idx;
 		ivideo->refresh_rate = 60;