Use fail-safe ReadBit overload in join packet parser

[Dutchman101]

Use fail-safe ReadBit overload in join packet parser

The no-arg ReadBit() returns the bit value, not a success indicator - on
bitstream exhaustion it silently returns false, not distinguishable from
reading a 0 bit. Switch to the ReadBit(bool&) overload from bitstream.h
which wraps ReadBits internally and properly returns false on exhaustion.

Now, the ReadBit is consistent with every other read in the function, (they all early-return on failure)

Scenario where it (pre-PR) would be a problem, and how:

If the bitstream is exhausted:

1. ReadBit() returns false - indistinguishable from "the bit was 0"
2. The parser continues with m_bOptionalUpdateInfoRequired = false instead of failing
3. Subsequent reads operate on garbage/out-of-bounds data, and that will cause unexpected behavior and real problems.

[Dutchman101]

I've updated the desc with a plausible bug scenario