Fix #77020: null pointer dereference in imap_mail

If an empty $message is passed to imap_mail(), we must not set message
to NULL, since _php_imap_mail() is not supposed to handle NULL pointers
(opposed to pointers to NUL).

NEWS

@@ -3,6 +3,7 @@ PHP                                                                        NEWS
 ?? ??? 2018, PHP 5.6.39
 
 - IMAP:
+  . Fixed bug #77020 (null pointer dereference in imap_mail). (cmb)
   . Fixed bug #77153 (imap_open allows to run arbitrary shell commands via
     mailbox parameter). (Stas)
 

ext/imap/php_imap.c

@@ -4094,7 +4094,6 @@ PHP_FUNCTION(imap_mail)
 	if (!message_len) {
 		/* this is not really an error, so it is allowed. */
 		php_error_docref(NULL TSRMLS_CC, E_WARNING, "No message string in mail command");
-		message = NULL;
 	}
 
 	if (_php_imap_mail(to, subject, message, headers, cc, bcc, rpath TSRMLS_CC)) {

ext/imap/tests/bug77020.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Bug #77020 (null pointer dereference in imap_mail)
+--SKIPIF--
+<?php
+if (!extension_loaded('imap')) die('skip imap extension not available');
+?>
+--FILE--
+<?php
+imap_mail('1', 1, NULL);
+?>
+===DONE===
+--EXPECTF--
+Warning: imap_mail(): No message string in mail command in %s on line %d
+%s
+===DONE===