check-source: add check for tal_free(tmpctx).

My older rebased patches use this, and I want to catch it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -272,7 +272,10 @@ check-shellcheck:
 check-setup_locale:
 	@tools/check-setup_locale.sh
 
-check-source: check-makefile check-source-bolt check-whitespace check-markdown check-spelling check-python check-includes check-cppcheck check-shellcheck check-setup_locale
+check-tmpctx:
+	@if git grep -n 'tal_free[(]tmpctx)' | grep -Ev '^ccan/|/test/|^common/daemon.c:|^common/utils.c:'; then echo "Don't free tmpctx!">&2; exit 1; fi
+
+check-source: check-makefile check-source-bolt check-whitespace check-markdown check-spelling check-python check-includes check-cppcheck check-shellcheck check-setup_locale check-tmpctx
 
 full-check: check check-source