lavc/bmp: Avoid a heap buffer overwrite for 1bpp input.

Found by Mingi Cho, Seoyoung Kim, and Taekyoung Kwon
of the Information Security Lab, Yonsei University.

(cherry picked from commit 1e340140)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/bmp.c

@@ -291,7 +291,7 @@ static int bmp_decode_frame(AVCodecContext *avctx,
         case 1:
             for (i = 0; i < avctx->height; i++) {
                 int j;
-                for (j = 0; j < n; j++) {
+                for (j = 0; j < avctx->width >> 3; j++) {
                     ptr[j*8+0] =  buf[j] >> 7;
                     ptr[j*8+1] = (buf[j] >> 6) & 1;
                     ptr[j*8+2] = (buf[j] >> 5) & 1;
@@ -301,6 +301,9 @@ static int bmp_decode_frame(AVCodecContext *avctx,
                     ptr[j*8+6] = (buf[j] >> 1) & 1;
                     ptr[j*8+7] =  buf[j]       & 1;
                 }
+                for (j = 0; j < (avctx->width & 7); j++) {
+                    ptr[avctx->width - (avctx->width & 7) + j] = buf[avctx->width >> 3] >> (7 - j) & 1;
+                }
                 buf += n;
                 ptr += linesize;
             }