Fix segv on uninitialized command fields.

include/internal/parser/parser.h

@@ -65,10 +65,10 @@ struct AddressPromise {
 struct Command {
   bool temp_address{false};
   std::vector<AddressPromise> addresses{};
-  functions::Function *function;
-  functions::Function::Argument argument;
+  functions::Function *function{nullptr};
+  functions::Function::Argument argument{std::monostate()};
   std::vector<AddressPromise> argument_addresses{};
-  functions::Suffix *suffix;
+  functions::Suffix *suffix{nullptr};
 };
 
 struct CompletionContext {

src/bed/run.cc

@@ -124,7 +124,8 @@ void BEd::handle(std::string_view cmd, bool eof) {
         throw ed_error("Line number can't be zero.");
     }
   }
-  c.function->handle(*this, address, nullptr, c.argument, nullptr);
+  if (c.function->handle)
+    c.function->handle(*this, address, nullptr, c.argument, nullptr);
   if (c.suffix)
     c.suffix->handle(*this);
   if (c.temp_address)