[C++] Fix issues found by OSS-Fuzz in IPC reader

[pitrou]

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

Fix two issues found by OSS-Fuzz in the IPC reader:

• a controlled abort: https://oss-fuzz.com/testcase-detail/5301064831401984
• a nullptr dereference: https://oss-fuzz.com/testcase-detail/5091511766417408

None of these two issues is a security issue.

Component(s)

C++

[pitrou]

Issue resolved by pull request 49060
#49060