fix issue #10222: mythcommflag segfaulting

The problem in issue #10222 is that it is adding the CODEC_FLAG2_FAST flag.  This causes unsafe functions to be used, causing a segfault. The proper way to fix this would probably be to figure out how to test for the "experimental speedups" flag, however I don't know the codebase well enough for this.

I tested how this would affect the speed of mythcommflag on one of my recordings.  The results:

Before removing flag:
real 2m13.123s
user 2m16.713s
sys 0m3.502s

After removing flag:
real 2m13.021s
user 2m15.446s
sys 0m3.758s

Signed-off-by: Stuart Auchterlonie <stuarta@squashedfrog.net>
(cherry-picked from 0026dda883ef50ab7f2fda7f2d2218bc56f094a7)