util.c: fix off-by-one error

* util.c (ruby_scan_digits): fix the return length off-by-one
  error when the length is given and the last char is a digit.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50935 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

util.c

@@ -94,6 +94,7 @@ ruby_scan_digits(const char *str, ssize_t len, int base, size_t *retlen, int *ov
     do {
 	int d = ruby_digit36_to_number_table[(unsigned char)*str++];
         if (d == -1 || base <= d) {
+	    --str;
 	    break;
         }
         if (mul_overflow < ret)
@@ -104,7 +105,7 @@ ruby_scan_digits(const char *str, ssize_t len, int base, size_t *retlen, int *ov
         if (ret < x)
             *overflow = 1;
     } while (len < 0 || --len);
-    *retlen = (str-1) - start;
+    *retlen = str - start;
     return ret;
 }