Heap overflow via truncated base64 encoding of binary fields in length-limited JSON output

A memory-handling error in the BSON-to-JSON conversion helpers of the MongoDB C Driver can write a small number of bytes past the end of a heap buffer when a binary field is encoded and the output is cut short at a caller-configured length limit. A party who supplies the document content, with no privileges on the application that links the driver, may cause a small amount of data outside the intended buffer to be altered.