pbx: Prevent execution of NULL pointer.

pbx_extension_helper has a check for q->swo.exec == NULL but it doesn't
actually return so we would still run the function.  Fix the return.
Move the 'int res' variable into the only scope which uses it.

Also fix a copy-paste error in ast_pbx_init which could result in a
crash on allocation failure (we exit with a normal error instead).

Change-Id: I0693af921fdc7f56b6a72a21fb816ed08b960a69