bmv: fix apparent sign error in the frame_off check

Fixes part of Ticket1373
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/bmv.c

@@ -143,7 +143,7 @@ static int decode_bmv_frame(const uint8_t *source, int src_len, uint8_t *frame,
         switch (mode) {
         case 1:
             if (forward) {
-                if (dst - frame + SCREEN_WIDE < frame_off ||
+                if (dst - frame + SCREEN_WIDE < -frame_off ||
                         frame_end - dst < frame_off + len)
                     return -1;
                 for (i = 0; i < len; i++)
@@ -151,7 +151,7 @@ static int decode_bmv_frame(const uint8_t *source, int src_len, uint8_t *frame,
                 dst += len;
             } else {
                 dst -= len;
-                if (dst - frame + SCREEN_WIDE < frame_off ||
+                if (dst - frame + SCREEN_WIDE < -frame_off ||
                         frame_end - dst < frame_off + len)
                     return -1;
                 for (i = len - 1; i >= 0; i--)