string.c: fix integer overflow

* string.c (rb_str_subpos): fix integer overflow which can happen
  only when SHARABLE_MIDDLE_SUBSTRING is enabled.  incorpolate
  https://github.com/mruby/mruby/commit/7db0786abdd243ba031e24683f

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57797 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -2386,7 +2386,7 @@ rb_str_subpos(VALUE str, long beg, long *lenp)
 	    beg += blen;
 	    if (beg < 0) return 0;
 	}
-	if (beg + len > blen)
+	if (len > blen - beg)
 	    len = blen - beg;
 	if (len < 0) return 0;
 	p = s + beg;