Fix segfault on remove_method with invalid argument

Reported by https://hackerone.com/jpenalbae

src/class.c

@@ -2068,7 +2068,7 @@ mrb_mod_remove_method(mrb_state *mrb, mrb_value mod)
 
   mrb_get_args(mrb, "*", &argv, &argc);
   while (argc--) {
-    remove_method(mrb, mod, mrb_symbol(*argv));
+    remove_method(mrb, mod, to_sym(mrb, *argv));
     argv++;
   }
   return mod;

test/t/class.rb

@@ -401,3 +401,12 @@ assert('class with non-class/module outer raises TypeError') do
   assert_raise(TypeError) { class 0::C1; end }
   assert_raise(TypeError) { class []::C2; end }
 end
+
+assert("remove_method doesn't segfault if the passed in argument isn't a symbol") do
+  klass = Class.new
+  assert_raise(TypeError) { klass.remove_method nil }
+  assert_raise(TypeError) { klass.remove_method 123 }
+  assert_raise(TypeError) { klass.remove_method 1.23 }
+  assert_raise(NameError) { klass.remove_method "hello" }
+  assert_raise(TypeError) { klass.remove_method Class.new }
+end