bridge.c: NULL app causes crash during attended transfer

Due to a race condition there was a chance that during an attended transfer the
channel's application would return NULL. This, of course, would cause a crash
when attempting to access the memory. This patch retrieves the channel's app
at an earlier time in processing in hopes that the app name is available.
However, if it is not then "unknown" is used instead. Since some string value
is now always present the crash can no longer occur.

ASTERISK-24869 #close
Reported by: viniciusfontes
Review: https://gerrit.asterisk.org/#/c/133/

Change-Id: I5134b84c4524906d8148817719d76ffb306488ac