Clean up ast_check_digits

The code was originally copied from the is_int() function in the AEL
code. wdoekes pointed out that the function should take a const char*
and that their was an unneeded variable. This is now fixed.
........

Merged revisions 341529 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 341530 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@341533 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/strings.h

@@ -878,13 +878,13 @@ int __attribute__((format(printf, 3, 4))) ast_str_append(
  * \retval 0 The string contains non-digit characters
  */
 AST_INLINE_API(
-int ast_check_digits(char *arg),
+int ast_check_digits(const char *arg),
 {
-	char *s;
-	for (s=arg; *s; s++) {
-		if (*s < '0' || *s > '9') {
+	while (*arg) {
+		if (*arg < '0' || *arg > '9') {
 			return 0;
 		}
+		arg++;
 	}
 	return 1;
 }