Fix a bug whereby we were getting ~0 and comparing it to maxsize, i.e. if

s1 was 0 length, and replstr was 0 length, etc., we would end up subtracting
one from zero and seeing if it was greater than the size_t (unsigned) var
maxsize...  This would cause us to return a string consisting of essentially
only match, which is not the right behaviour if we have 0 length inpline.

usr.bin/xargs/strnsubst.c

@@ -48,7 +48,7 @@ strnsubst(char **str, const char *match, const char *replstr, size_t maxsize)
 		if (this == NULL)
 			break;
 		if ((strlen(s2) + ((uintptr_t)this - (uintptr_t)s1) +
-		    (strlen(replstr) - 1)) > maxsize) {
+		    (strlen(replstr) - 1)) > maxsize && *replstr != '\0') {
 			strlcat(s2, s1, maxsize);
 			goto done;
 		}