Fix segfault while updating ${.sh.match}

The SHOPT_2DMATCH code block in sh_setmatch() modifies the 'ap'
pointer, which is initialised as nv_arrayptr(SH_MATCHNOD). This
caused a (rarely occurring) segfault in the following line near the
end of the function:
	ap->nelem -= x;
as this line assumed that 'ap' still had the initial value.

src/cmd/ksh93/sh/init.c: sh_setmatch():
- On init, save ap in ap_save and use ap_save instead of ap where
  it should be pointing to SH_MATCHNOD. This also allows removing
  two redundant nv_arrayptr(SH_MATCHNOD) calls, slightly increasing
  the efficiency of this function.

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-01-07:
+
+- Fixed a crash that could occur while ksh updated ${.sh.match}.
+
 2021-01-05:
 
 - Fixed a bug in 'cd' that caused 'cd ./foo' to search for 'foo' in $CDPATH.