Fix _from argument validation

program/steps/mail/sendmail.inc

@@ -101,10 +101,13 @@ if (is_numeric($from)) {
     }
 }
 // ... if there is no identity record, this might be a custom from
-else if ($from_string = rcmail_email_input_format($from)) {
-    if (preg_match('/(\S+@\S+)/', $from_string, $m))
+else if (($from_string = rcmail_email_input_format($from))
+    && preg_match('/(\S+@\S+)/', $from_string, $m)
+) {
     $from = trim($m[1], '<>');
-    else
+}
+// ... otherwise it's empty or invalid
+else {
     $from = null;
 }