fix label followed by a declaration and add more tests

Author: jordikroon

ext/standard/basic_functions.c

@@ -1350,11 +1350,15 @@ PHPAPI zend_result _php_error_log(int opt_err, const zend_string *message, const
 	php_stream *stream = NULL;
 	size_t nbytes;
 
+	const char *to = NULL;
+	const char *hdrs = NULL;
+
 	switch (opt_err)
 	{
 		case 1:		/*send an email */
-			const char *to = opt ? ZSTR_VAL(opt) : NULL;
-			const char *hdrs = headers ? ZSTR_VAL(headers) : NULL;
+			to = opt ? ZSTR_VAL(opt) : NULL;
+			hdrs = headers ? ZSTR_VAL(headers) : NULL;
+
 			if (!php_mail(to, "PHP error_log message", ZSTR_VAL(message), hdrs, NULL)) {
 				return FAILURE;
 			}

tests/basic/gh20858_headers.phpt

@@ -0,0 +1,19 @@
+--TEST--
+GH-20858 Null pointer dereference in php_mail_detect_multiple_crlf via error_log
+--FILE--
+<?php
+
+$headers  = "From: test <[email protected]>\n";
+$headers .= "Cc: test <[email protected]>\n"; 
+$headers .= 'X-Mailer: PHP/' . phpversion();
+
+// Send mail with destination set
+error_log(0, 1, "[email protected]>", null);
+
+// Send mail with custom headers
+error_log(0, 1, null, $headers);
+
+// Send mail with destination set + custom headers
+error_log(0, 1, "[email protected]>", $headers);
+?>
+--EXPECTF--