rational.c: initialize n

* rational.c (read_num): `n` was used uninitialized when the
  string started with a period.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57991 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

rational.c

@@ -2280,7 +2280,7 @@ islettere(int c)
 static int
 read_num(const char **s, const char *const end, VALUE *num, VALUE *div)
 {
-    VALUE fp = ONE, exp, fn = ZERO, n;
+    VALUE fp = ONE, exp, fn = ZERO, n = ZERO;
     int expsign = 0, ok = 0;
     char *e;