Don't leak header callback if headers already sent

We need to avoid storing it in the first place, as we don't
really have a good place to release it later. If headers haven't
been sent yet, send_headers will do this. sapi_deactive happens
too late in the shutdown sequence and will result in leak reports.

main/SAPI.c

@@ -124,7 +124,11 @@ PHP_FUNCTION(header_register_callback)
 		SG(fci_cache) = empty_fcall_info_cache;
 	}
 
-	ZVAL_COPY(&SG(callback_func), &fci.function_name);
+	/* Don't store callback if headers have already been sent:
+	 * It won't get used and we won't have a chance to release it. */
+	if (!SG(headers_sent)) {
+		ZVAL_COPY(&SG(callback_func), &fci.function_name);
+	}
 
 	RETURN_TRUE;
 }

tests/basic/header_register_callback_after_output.phpt

@@ -0,0 +1,9 @@
+--TEST--
+Call header_register_callback() after headers sent
+--FILE--
+<?php
+echo "Send headers.\n";
+header_register_callback(function() { echo "Too late";});
+?>
+--EXPECT--
+Send headers.