Fix two more 'command' bugs

BUG 1: Though 'command' is specified/documented as a regular
builtin, preceding assignments survive the invocation (as with
special or declaration builtins) if 'command' has no command
arguments in these cases:

$ foo=wrong1 command; echo $foo
wrong1
$ foo=wrong2 command -p; echo $foo
wrong2
$ foo=wrong3 command -x; echo $foo
wrong3

Analysis: sh_exec(), case TCOM (simple command), contains the
following loop that skips over 'command' prefixes, preparsing any
options and remembering the offset in the 'command' variable:

src/cmd/ksh93/sh/xec.c
1059 while(np==SYSCOMMAND || !np && com0
     && nv_search(com0,shp->fun_tree,0)==SYSCOMMAND)
1060 {
1061         register int n = b_command(0,com,&shp->bltindata);
1062         if(n==0)
1063                 break;
1064         command += n;
1065         np = 0;
1066         if(!(com0= *(com+=n)))
1067                 break;
1068         np = nv_bfsearch(com0, shp->bltin_tree, &nq, &cp);
1069 }

This skipping is not done if the preliminary b_command() call on
line 1061 (with argc==0) returns zero. This is currently the case
for command -v/-V, so that 'command' is treated as a plain and
regular builtin for those options.

The cause of the bug is that this skipping is even done if
'command' has no arguments. So something like 'foo=bar command' is
treated as simply 'foo=bar', which of course survives.

So the fix is for b_command() to return zero if there are no
arguments. Then b_command() itself needs changing to not error out
on the second/main b_command() call if there are no arguments.

src/cmd/ksh93/bltins/whence.c: b_command():
- When called with argc==0, return a zero offset not just for -v
  (X_FLAG) or -V (V_FLAG), but also if there are no arguments left
  (!*argv) after parsing options.
- When called with argc>0, do not issue a usage error if there are
  no arguments, but instead return status 0 (or, if -v/-V was given,
  status 2 which was the status of the previous usage message).
  This way, 'command -v $emptyvar' now also works as you'd expect.

BUG 2: 'command -p' sometimes failed after executing certain loops.

src/cmd/ksh93/sh/path.c: defpath_init():
- astconf() returns a pointer to memory that may be overwritten
  later, so duplicate the string returned. Backported from ksh2020.
  (re: f485fe0f, aa4669ad, <https://github.com/att/ast/issues/959>)

src/cmd/ksh93/tests/builtins.sh:
- Update the test for BUG_CMDSPASGN to check every variant of
  'command' (all options and none; invoking/querying all kinds of
  command and none) with a preceding assignment. (re: fae8862c)
  This also covers bug 2 as 'command -p' was failing on macOS prior
  to the fix due to a loop executed earlier in another test.

NEWS

@@ -3,6 +3,15 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-05-05:
+
+- Fixed: a preceding variable assignment like foo=bar in 'foo=bar command'
+  (with no command arguments after 'command') incorrectly survived the
+  'command' regular built-in command invocation.
+
+- Fixed: 'command -p some_utility' intermittently failed to find the utility
+  under certain conditions due to a memory corruption issue.
+
 2021-05-03:
 
 - Subshells (even if non-forked) now keep a properly separated state of the