remoteip: avoid pointer underflow on empty header token

[mcc0nnell]

remoteip_modify_request() trims each RemoteIPHeader token by forming
parse_remote + strlen(parse_remote) - 1 before checking whether the token is
empty. A trailing comma can make the token empty, for example:

X-Forwarded-For: 192.0.2.1,

In that case the old code forms a pointer one byte before the string object.
ASan invalid-pointer-pair instrumentation catches the old form; the
end-exclusive form in this patch is clean.

Use an end-exclusive pointer and inspect eos[-1] only after eos > parse_remote. This keeps the existing literal-space trimming semantics and
avoids forming the invalid pointer.

The regression test exercises a trailing empty RemoteIPHeader token through
mod_remoteip.

Validation:

• git diff --check
• Python test module compiles
• patched mod_remoteip builds as a DSO with apxs
• throwaway httpd config loads the patched module and returns 200 for the
  trailing-empty-token request
• ASan detect_invalid_pointer_pairs=2: old primitive reports an invalid
  pointer pair; new end-exclusive primitive exits cleanly

[notroj]

LGTM apart from the test ordering nit.

[notroj]

I just noticed this (and the previous test added here) are not in sorted order in this file, can you reorder please?

[notroj]

Thank you!