Avoid dereferencing an undefined pointer in DecodeInterval().

Avoid dereferencing an undefined pointer in DecodeInterval().

Commit e39f99046 moved some code up closer to the start of
DecodeInterval(), without noticing that it had been implicitly
relying on previous checks to reject the case of empty input.
Given empty input, we'd now dereference a pointer that hadn't been
set, possibly leading to a core dump.  (But if we fail to provoke
a SIGSEGV, nothing bad happens, and the expected syntax error is
thrown a bit later.)

Per bug #17788 from Alexander Lakhin.  Back-patch to v15 where
the fault was introduced.

Discussion: https://postgr.es/m/17788-dabac9f98f7eafd5@postgresql.org