Backpatch to 7.4 the part of 1.84 (from the 8.0 timeline) that wasn't already

Backpatch to 7.4 the part of 1.84 (from the 8.0 timeline) that wasn't already
patched, viz. str_numth().  The rest of that patch was already applied as part
of 1.69.2.1.  Per report and patch from Andreas 'ads' Scherbaum.

The involved revisions were:

revision 1.84
date: 2005-01-12 22:40:13 -0300;  author: tgl;  state: Exp;  lines: +9 -7;
branches:  1.84.4;
Remove unportable assumption that it's okay to use the target buffer
of an sprintf() as a source string.  Demonstrably does not work with
recent gcc and/or glibc on some platforms.

and

revision 1.69.2.1
date: 2005-03-25 20:42:21 -0400;  author: tgl;  state: Exp;  lines: +24 -8;
Prevent to_char(interval) from dumping core on month-related formats
when a zero-month interval is given.  Per discussion with Karel.