avconv: add an assert to silence an uninitialized variable warning.

The warning silenced was:
avconv.c: In function ‘opt_output_file’:
avconv.c:3380:21: warning: ‘meta_out’ may be used uninitialized in this function [-Wuninitialized]
avconv.c:3315:20: note: ‘meta_out’ was declared here
