When looking up environment variables, boost process assumes the keys are of equal size

This causes a heap-buffer-overflow detected by ASAN
https://godbolt.org/z/Eno9r737M

https://github.com/boostorg/process/blob/develop/include/boost/process/environment.hpp#L263-L269

Start with include/boost/process/environment.hpp around lines 263-269 and reproduce the heap-buffer-overflow using the linked Godbolt ASAN example. Trace the environment-variable lookup with keys of unequal sizes. Done means the lookup no longer triggers the reported overflow under ASAN.

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