Fix null key dereference in post_iterator (#375)

[etr]

Fixes #375.

Problem

MHD may invoke webserver::post_iterator with a null key on a continuation chunk (off > 0) — the field name is supplied only on the first callback and not repeated when a field is split across callbacks (the large-field path from #337 / commit 1b5fe8f).

The no-file branch passed the raw key pointer straight into std::string (via grow_last_arg / set_arg), which throws std::logic_error on null. Because the throw escapes the C post-iterator callback, it propagates as an uncaught exception and aborts the process via std::terminate.

Fix

Guard key for null before constructing a std::string. With no field name there is nothing to store the value under, so accept and silently skip the chunk: MHD_YES tells MHD to continue (MHD_NO would abort the whole request). This is the same class of bug — and the same fix shape — as the null-uri guard in uri_log (#371).

Testing

webserver::post_iterator is a private static member with no test seam (unlike uri_log, which is a free function the unit test re-declares), so this change is covered by the existing file_upload integration test — its 192 checks exercise the normal multipart/form POST path and stay green, confirming no regression to legitimate form/file handling. A dedicated null-key regression unit test ships on the feature/v2.0 branch (#376), where post_iterator was refactored into a publicly testable static on webserver_impl.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Rkuh4aSmrD8m2f2vYqakb6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.07%. Comparing base (8b6aeb0) to head (1f59143).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
+ Coverage   68.03%   68.07%   +0.03%     
==========================================
  Files          34       34              
  Lines        1730     1732       +2     
  Branches      697      698       +1     
==========================================
+ Hits         1177     1179       +2     
  Misses         80       80              
  Partials      473      473              

Files with missing lines	Coverage Δ
src/webserver.cpp	53.73% <100.00%> (+0.15%)	⬆️

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