c_str() / data() lifetime issues in generated targets

[oliverchang]

Some generated targets have the following bad pattern:

const char *server = stream.ConsumeRemainingBytesAsString().c_str();

The std::string returned by FuzzedDataProvider instantly goes out of scope, leading to UAF on use of the pointer.

[oliverchang]

We can potentially detect instances of this (and potentially more general issues) by checking if the ASan-reported free stack has LLVMFuzzerTestOneInput at the top, which indicates the freeing happened in the target.