Fix heap buffer overflow in AP4_Ac4Parser and AP4_Dac4Atom

[MingxinChen]

Changes:

• Added GetBitsAvailable() method to AP4_BitReader to expose the number of bits remaining in the buffer
• Added bounds checking with appropriate error handling to prevent out-of-bounds reads when parsing malformed/maliciously crafted MP4 files

Related Issues: #1060, #1059, #1058