[Swift] Initialize memory when clear ByteBuffer (#5982)

* Initialize memory when clear ByteBuffer

It seems (based on my limited understanding) that FlatBuffers requires
the writable area to be 0 initialized. However, we missed it when clear
the buffer to reinitialize it.

This PR fixed that by calling initialize (also fixed the typo)
explicitly.

* Update version to 0.5.3

swift/FlatBuffers.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'FlatBuffers'
-  s.version          = '0.5.2'
+  s.version          = '0.5.3'
   s.summary          = 'FlatBuffers: Memory Efficient Serialization Library'
 
   s.description      = "FlatBuffers is a cross platform serialization library architected for