Make tests pass when --disable-decnum

tests/shtest

@@ -668,9 +668,11 @@ if ! x=$($JQ -cn '[
 fi
 
 # CVE-2023-50268: No stack overflow comparing a nan with a large payload
-$VALGRIND $Q $JQ '1 != .' <<\EOF >/dev/null
-Nan4000
+if $JQ -ne 'have_decnum'; then
+  $VALGRIND $Q $JQ '1 != .' <<\EOF >/dev/null
+  Nan4000
 EOF
+fi
 
 # Allow passing the inline jq script before -- #2919
 if ! r=$($JQ --args -rn -- '$ARGS.positional[0]' bar) || [ "$r" != bar ]; then