bind/objc: fix incorrect objc reference handling in RefTracker

The objc reference handling has bug on assigning different refnum
to the same ios object. The reason for this is that the
RefTracker _refs is not properly initialized thus incorrect
reference check in assignRefnumAndIncRefcount.

Change-Id: Id86423dcf378d11e9056bf7c7ecb646333a94a04
Reviewed-on: https://go-review.googlesource.com/21120
Reviewed-by: Elias Naur <elias.naur@gmail.com>

bind/objc/seq_darwin.m.support

@@ -270,6 +270,7 @@ nstring go_seq_from_objc_string(NSString *s) {
   self = [super init];
   if (self) {
     _next = 42;
+    _refs = [[NSMutableDictionary alloc] init];
     _objs = [[NSMutableDictionary alloc] init];
   }
   return self;