eval.c: suppress a warning

* eval.c (setup_exception): suppress a maybe-uninitialized false
  warning by gcc 4.8.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46566 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

eval.c

@@ -471,7 +471,7 @@ setup_exception(rb_thread_t *th, int tag, volatile VALUE mesg, VALUE cause)
 {
     VALUE at;
     VALUE e;
-    const char *file;
+    const char *file = 0;
     volatile int line = 0;
     int nocause = 0;