pgsql: Fix pg_strtof() to not crash on NULL endptr.

Fix pg_strtof() to not crash on NULL endptr.

We had managed not to notice this simple oversight because none
of our calls exercised the case --- until commit 8f427187d.
That led to pg_dump crashing on any platform that uses this code
(currently Cygwin and Mingw).

Even though there's no immediate bug in the back branches, backpatch,
because a non-POSIX-compliant strtof() substitute is trouble waiting
to happen for extensions or future back-patches.

Diagnosed-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/339b3902-4e98-4e31-a744-94e43b7b9292@gmail.com
Backpatch-through: 13

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c7303f01c574c3543c68452c7dfd8998efe25085

Modified Files
--------------
src/port/strtof.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)