ffserver: Check chunk size

Fixes out of array access

Fixes: poc_ffserver.py
Found-by: Paul Cher <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a5d25faa3f)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffserver.c

@@ -2702,8 +2702,10 @@ static int http_receive_data(HTTPContext *c)
         } else if (c->buffer_ptr - c->buffer >= 2 &&
                    !memcmp(c->buffer_ptr - 1, "\r\n", 2)) {
             c->chunk_size = strtol(c->buffer, 0, 16);
-            if (c->chunk_size == 0) // end of stream
+            if (c->chunk_size <= 0) { // end of stream or invalid chunk size
+                c->chunk_size = 0;
                 goto fail;
+            }
             c->buffer_ptr = c->buffer;
             break;
         } else if (++loop_run > 10)
@@ -2725,6 +2727,7 @@ static int http_receive_data(HTTPContext *c)
             /* end of connection : close it */
             goto fail;
         else {
+            av_assert0(len <= c->chunk_size);
             c->chunk_size -= len;
             c->buffer_ptr += len;
             c->data_count += len;