Fix security issue with PB_ENABLE_MALLOC.

The multiplication in allocate_field could potentially overflow,
leading to allocating too little memory. This could subsequently
allow an attacker to cause a write past the buffer, overwriting
other memory contents.

The attack is possible if untrusted message data is decoded using
nanopb, and the message type includes a pointer-type string or bytes
field, or a repeated numeric field. Submessage fields are not
affected.

This issue only affects systems that have been compiled with
PB_ENABLE_MALLOC enabled. Only version nanopb-0.2.7 is affected,
as prior versions do not include this functionality.

Update issue 117
Status: FixedInGit