[C++] Make IPC message decoding stricter #50240

Describe the bug, including details regarding any error messages, version, and platform.

Currently, when decoding an IPC message from an IPC file, we don't notice if the advertised metadata size is larger than the actual serialized metadata size.

We should make it an error to guard against invalid IPC files (for example with erroneous padding).

Found by OSS-Fuzz in https://issues.oss-fuzz.com/issues/524437775

Component(s)

C++