Fix null key dereference in post_iterator (#375)

MHD may invoke the post iterator with a null key on a continuation
chunk (off > 0): the field name is supplied only on the first call and
not repeated. The no-file branch passed the raw key pointer 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.

Guard key for null before constructing std::string: with no field name
there is nothing to store the value under, so accept and silently skip
the chunk (MHD_YES keeps the request alive; MHD_NO would abort it).
Same class of bug as the null-uri fix in uri_log (#371).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rkuh4aSmrD8m2f2vYqakb6

Author: etr

src/webserver.cpp

@@ -832,6 +832,20 @@ MHD_Result webserver::post_iterator(void *cls, enum MHD_ValueKind kind,
     struct details::modded_request* mr = (struct details::modded_request*) cls;
 
     if (!filename) {
+        // MHD may invoke the post iterator with a null key on a
+        // continuation chunk (off > 0): the field name was supplied on the
+        // first call and is not repeated. With no field name there is
+        // nothing to store the value under, so silently accept the chunk
+        // (MHD_YES tells MHD to continue; MHD_NO would abort the whole
+        // request). Guarding here also stops the raw pointer from reaching
+        // std::string, which throws std::logic_error on null and aborts the
+        // process via std::terminate because the throw escapes a C
+        // callback. See issue #375 (same class of bug as the null-uri fix
+        // in uri_log, issue #371).
+        if (!key) {
+            return MHD_YES;
+        }
+
         // There is no actual file, just set the arg key/value and return.
         if (off > 0) {
             mr->dhr->grow_last_arg(key, std::string(data, size));