parser: fix large overreads

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

libavcodec/parser.c

@@ -254,7 +254,9 @@ int ff_combine_frame(ParseContext *pc, int next, const uint8_t **buf, int *buf_s
         if(!new_buffer)
             return AVERROR(ENOMEM);
         pc->buffer = new_buffer;
-        memcpy(&pc->buffer[pc->index], *buf, next + FF_INPUT_BUFFER_PADDING_SIZE );
+        if (next > -FF_INPUT_BUFFER_PADDING_SIZE)
+            memcpy(&pc->buffer[pc->index], *buf,
+                   next + FF_INPUT_BUFFER_PADDING_SIZE);
         pc->index = 0;
         *buf= pc->buffer;
     }