libbb: shrink last_char_is(), no longer allow NULL string argument

function                                             old     new   delta
last_char_is                                          40      28     -12

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/last_char_is.c

@@ -11,14 +11,9 @@
 /* Find out if the last character of a string matches the one given */
 char* FAST_FUNC last_char_is(const char *s, int c)
 {
-	if (s) {
-		size_t sz = strlen(s);
-		/* Don't underrun the buffer if the string length is 0 */
-		if (sz != 0) {
-			s += sz - 1;
-			if ((unsigned char)*s == c)
-				return (char*)s;
-		}
-	}
-	return NULL;
+	if (!s[0])
+		return NULL;
+	while (s[1])
+		s++;
+	return (*s == (char)c) ? (char *) s : NULL;
 }