bugfix: immediate return from PG_TRY() .. PG_CATCH() block breaks exception stack

Likely, it leads system crash with uncertain stack trace.

  (gdb) bt
  #0  0x00007fe5e60ec7fe in __longjmp () from /lib64/libc.so.6
  #1  0xe803de6ca08457cd in ?? ()
  Cannot access memory at address 0x93dfde6c9f764523

Once PG_TRY() set PG_exception_stack a local pointer, code is
responsible to restore it prior to breaking.