Fix corner case bug in numeric to_char() some more.

Fix corner case bug in numeric to_char() some more.

The band-aid applied in commit f0bedf3e4 turns out to still need
some work: it made sure we didn't set Np->last_relevant too small
(to the left of the decimal point), but it didn't prevent setting
it too large (off the end of the partially-converted string).
This could result in fetching data beyond the end of the allocated
space, which with very bad luck could cause a SIGSEGV, though
I don't see any hazard of interesting memory disclosure.

Per bug #17839 from Thiago Nunes.  The bug's pretty ancient,
so back-patch to all supported versions.

Discussion: https://postgr.es/m/17839-aada50db24d7b0da@postgresql.org