mruby-sprintf: fix a bug with negative octals

Leading octal digit may be 1, 3, 7 unlike binary or hexadecimal.

mrbgems/mruby-sprintf/src/sprintf.c

@@ -666,6 +666,9 @@ retry:
         }
 
         if (dots) {
+          if (base == 8 && (*s == '1' || *s == '3')) {
+            s++; len--;
+          }
           while (*s == fc) {
             s++; len--;
           }