mpegtsenc: Don't pass NULL to memcpy

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/mpegtsenc.c

@@ -277,6 +277,10 @@ static void putstr8(uint8_t **q_ptr, const char *str, int write_len)
         len = strlen(str);
     if (write_len)
         *q++ = len;
+    if (!str) {
+        *q_ptr = q;
+        return;
+    }
     memcpy(q, str, len);
     q     += len;
     *q_ptr = q;