cmdutils_read_file: increment *size after writing the trailing \0

Fixes CID732166.

cmdutils.c

@@ -1061,7 +1061,7 @@ int cmdutils_read_file(const char *filename, char **bufptr, size_t *size)
             ret = AVERROR_EOF;
     } else {
         ret = 0;
-        (*bufptr)[*size++] = '\0';
+        (*bufptr)[(*size)++] = '\0';
     }
 
     fclose(f);