smc: fix the bounds check

Fixes invalid writes when there are more blocks in a run than total
remaining blocks.

CC: libav-stable@libav.org
Bug-ID: CVE-2014-8548
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit d423dd72be)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/smc.c

@@ -70,7 +70,7 @@ typedef struct SmcContext {
         row_ptr += stride * 4; \
     } \
     total_blocks--; \
-    if (total_blocks < 0) \
+    if (total_blocks < !!n_blocks) \
     { \
         av_log(s->avctx, AV_LOG_INFO, "warning: block counter just went negative (this should not happen)\n"); \
         return; \