Size entity attribute storage to schema arity, not token count

When a STEP instance has fewer attribute tokens than its schema declares
(commonly from corrupted/malformed syntax), parse_context::construct()
sized the in-memory attribute storage to the smaller token count instead
of the schema's attribute count. This left the storage's last N attribute
slots simply nonexistent rather than blank, so any later read of one of
those trailing attributes by index threw an uncaught IfcParse::IfcException
("Index N is out of range for storage of size N") that terminated the
whole process (SIGABRT) instead of being handled as a parse warning.

Fix: when the schema declaration is known, size the storage to the
schema's attribute count. Indices beyond the number of tokens found are
left at their existing default-constructed blank value (the storage
constructor already blank-initializes every slot), so a truncated
instance now degrades to blank values for its missing trailing
attributes, matching the parser's existing "expected N attribute values,
found M" warning intent instead of crashing.

Reproduced with the fuzzing script attached to #5679: single-byte
mutations of a minimal IFC4 file that corrupt the IFCPROJECT instance's
token stream reliably aborted IfcConvert with this exact exception before
the fix, and now parse with a logged syntax error and exit code 0.

Fixes #5679

Generated with the assistance of an AI coding tool.

src/ifcparse/IfcFile.cpp

@@ -273,9 +273,18 @@ IfcEntityInstanceData IfcParse::parse_context::construct(boost::optional<size_t>
         return IfcEntityInstanceData(in_memory_attribute_storage(0));
     }
 
+    // When the schema declaration is known, size the storage to the schema's
+    // attribute count rather than the (possibly smaller) number of tokens
+    // actually found. Attributes are only assigned for indices covered by
+    // tokens_ below; any remaining trailing indices stay at their
+    // default-constructed blank value. This keeps every instance's storage
+    // consistent with its schema arity, so that a malformed/truncated
+    // instance (e.g. corrupted STEP syntax dropping a trailing attribute)
+    // degrades to a blank value for the missing attribute instead of an
+    // out-of-range access when that attribute is later read by index.
     in_memory_attribute_storage storage(coerce_attribute_count
         ? (decl != nullptr
-        ? (std::min)(parameter_types.size(), tokens_.size())
+        ? parameter_types.size()
         : tokens_.size())
 		: tokens_.size()
     );