Try harder to avoid false positives for DV probe.

Try harder to avoid false positives for DV probe.
Require at least one signature match per provided 1MB of probe data,
and if there is only a single match, return at most MAX/4.
Fixes issue1382 but could/should probably still be improved.

Originally committed as revision 19848 to svn://svn.ffmpeg.org/ffmpeg/trunk