Correction to commit 0ba59aa6 using the correct variable in tel uri param parsing for mod_sofia

add heartbeat_use_scheduler variable to force the heartbeat to use the scheduler rather than the media stream and also make it auto-chage between modes when going in and out of the media path

FS-4779 FS-6467 the behaviour from 4479 is now masked behind the param 3pcc-reinvite-bridged-on-ack FS-6467 please test master

This seems to be a paradox when running a perl script from a session then executing perl again on the same session from a different thread.
I fixed it by converting any execution of perl in the execute_on_* family of operators to only run background mode which is to store the command in the session stack to be executed only by the session thread instead of on the spot by the outide thread.  changing the execute_on_answer to perl::/path/to/script.pl would also eliminate the crash in code that has not been updated with this patch.

This is just a limitation of embedded perl we have to live with.

Previous commit actually broke the functionality since it was now returning if cfg->path[0] was not null.
Also since cfg->path can never be null, this check can be simplified to only check the first index.

we only call this with values, and it would crash if you passed it null anyways, just assert to make it 100% clear

Fix buffer overrun that could have been exploited with a crafted skinny packet designed to exploit this mistake in bounds checking.