Make the behavior of functions more explicit.

Avoid creating a stringstream by using find().

Also declare the function static and warn if return value not used.

Prevent overflows in the addtion of q and size, and avoid truncations in
callers by using size_t as type for size.

Enable common additional compiler warnings.

Abort on truncation of values being written to the ELF data, to prevent
silent behavior mismatch.

Use auto to avoid implicit type conversion, hiding possible value
truncation.

Reject ELF data that would lead to invalid memory access or integer
overflows.