Tracking PR #1291 to fix two possible conditions which could result in a potential denial of service of a client connected to a malicious MongoDB server.

1. readLengthBytes requires 4 bytes for the length to be included. Previously when reading a document from the wire this could result in a tight loop where an empty struct is appended to a slice repeatedly until the service runs out of memory (both CPU and memory consumption).
2. Fix a large memory allocation condition with Snappy decompression if a large size is encoded in the Snappy compressed / encoded portion of the bytes.