sh_setmatch: do not allocate a size 0 block (re: f38494e)

In init.c:867, calloc(3) may be called with a size of 0 on line 867
if nmatch is zero but v (value) is non-NULL. (This is currently
triggered by the arrays.sh and attributes.sh regression tests.)

As POSIX states: "If the size of the space requested is 0, the
behavior is implementation-defined: either a null pointer shall be
returned, or the behavior shall be as if the size were some
non-zero value, except that the behavior is undefined if the
returned pointer is used to access an object."[*] Either way, the
behaviour is undefined if the returned pointer is dereferenced.

src/cmd/ksh93/sh/init.c: sh_set_match():
- Before calling calloc, return if either nmatch is 0 (there were
  no matches) or v is NULL (there is no value). Do not require both
  conditions to be true to return.

[*] https://pubs.opengroup.org/onlinepubs/9799919799/functions/calloc.html

src/cmd/ksh93/sh/init.c

@@ -830,7 +830,7 @@ void sh_setmatch(const char *v, int vsize, int nmatch, int match[], int index)
 			free(ap);
 		SH_MATCHNOD->nvalue = NULL;
 		SH_MATCHNOD->nvfun = NULL;
-		if(!(mp->nmatch=nmatch) && !v)
+		if(!(mp->nmatch=nmatch) || !v)
 		{
 			sh.subshell = savesub;
 			return;