Fix a bug in av_find_opt(). Because some of the AVOption structures have field

Fix a bug in av_find_opt(). Because some of the AVOption structures have field
unit = NULL, the function could pass NULL to strcmp and cause a segfault.

Patch by Kamil Nowosad, k nowosad % students mimuw edu pl.

Original thread:
Subject: [PATCH] small bugfix in av_find_opt()
Date: 03/23/2007 12:20 PM

Originally committed as revision 8553 to svn://svn.ffmpeg.org/ffmpeg/trunk