pbx_variables.c: Prevent SEGV due to stack overflow.

pbx_variables.c: Prevent SEGV due to stack overflow.

It is possible for dialplan to result in an infinite
recursion of variable substitution, which eventually
leads to stack overflow. If we detect this, abort
substitution and log an error for the user to fix
the broken dialplan.

Resolves: #480

UpgradeNote: The maximum amount of dialplan recursion
using variable substitution (such as by using EVAL_EXTEN)
is capped at 15.