res_pjsip: Fix crash with from_user containing invalid characters.

res_pjsip: Fix crash with from_user containing invalid characters.

If the from_user field contains certain characters (like @, {, ^, etc.),
PJSIP will return a null value for the URI when attempting to parse it.
This causes a crash when trying to dial out through a trunk that contains
these invalid characters in its from_user field.

This change checks the configuration and ensures that an endpoint will
not be created if the from_user contains an invalid character. It also
adds a null check to the PJSIP URI parsing as a backup.

ASTERISK-27036 #close
Reported by: Maxim Vasilev

Change-Id: I0396fdb5080604e0bdf1277464d5c8a85db913d0