Fix some cases of reading of 64bit map values.

Fixes https://github.com/google/protobuf/issues/3164.

objectivec/GPBDictionary.m

@@ -406,7 +406,7 @@ static void ReadValue(GPBCodedInputStream *stream,
       valueToFill->valueInt32 = GPBCodedInputStreamReadInt32(&stream->state_);
       break;
     case GPBDataTypeInt64:
-      valueToFill->valueInt64 = GPBCodedInputStreamReadInt32(&stream->state_);
+      valueToFill->valueInt64 = GPBCodedInputStreamReadInt64(&stream->state_);
       break;
     case GPBDataTypeSInt32:
       valueToFill->valueInt32 = GPBCodedInputStreamReadSInt32(&stream->state_);

objectivec/Tests/GPBMessageTests+Serialization.m

@@ -1149,22 +1149,27 @@ static NSData *DataFromCStr(const char *str) {
   [msg.mapInt32Int32 setInt32:101 forKey:2001];
   [msg.mapInt64Int64 setInt64:1002 forKey:202];
   [msg.mapInt64Int64 setInt64:103 forKey:2003];
+  [msg.mapInt64Int64 setInt64:4294967296 forKey:4294967297];
   [msg.mapUint32Uint32 setUInt32:1004 forKey:204];
   [msg.mapUint32Uint32 setUInt32:105 forKey:2005];
   [msg.mapUint64Uint64 setUInt64:1006 forKey:206];
   [msg.mapUint64Uint64 setUInt64:107 forKey:2007];
+  [msg.mapUint64Uint64 setUInt64:4294967298 forKey:4294967299];
   [msg.mapSint32Sint32 setInt32:1008 forKey:208];
   [msg.mapSint32Sint32 setInt32:109 forKey:2009];
   [msg.mapSint64Sint64 setInt64:1010 forKey:210];
   [msg.mapSint64Sint64 setInt64:111 forKey:2011];
+  [msg.mapSint64Sint64 setInt64:4294967300 forKey:4294967301];
   [msg.mapFixed32Fixed32 setUInt32:1012 forKey:212];
   [msg.mapFixed32Fixed32 setUInt32:113 forKey:2013];
   [msg.mapFixed64Fixed64 setUInt64:1014 forKey:214];
   [msg.mapFixed64Fixed64 setUInt64:115 forKey:2015];
+  [msg.mapFixed64Fixed64 setUInt64:4294967302 forKey:4294967303];
   [msg.mapSfixed32Sfixed32 setInt32:1016 forKey:216];
   [msg.mapSfixed32Sfixed32 setInt32:117 forKey:2017];
   [msg.mapSfixed64Sfixed64 setInt64:1018 forKey:218];
   [msg.mapSfixed64Sfixed64 setInt64:119 forKey:2019];
+  [msg.mapSfixed64Sfixed64 setInt64:4294967304 forKey:4294967305];
   [msg.mapInt32Float setFloat:1020.f forKey:220];
   [msg.mapInt32Float setFloat:121.f forKey:2021];
   [msg.mapInt32Double setDouble:1022. forKey:222];