Optimize out strlen call

As the buffer max size is known, can check last byte with isset

Signed-off-by: Elan Ruusamäe <[email protected]>

src/Protocol/Smtp.php

@@ -331,7 +331,7 @@ public function data($data)
                 $line .= $buffer;
 
                 // partial read, continue loop to read again to complete the line
-                if (strlen($buffer) === $chunkSize - 1 && $buffer[$chunkSize - 2] !== "\n") {
+                if (isset($buffer[$chunkSize - 2]) && $buffer[$chunkSize - 2] !== "\n") {
                     continue;
                 }