Fix Undefined-shift in pcpp::SmtpRequestLayer::getCommand (#1553)

* Fix Undefined-shift in pcpp::SmtpRequestLayer::getCommand

* update

Author: egecetin

Packet++/src/SmtpLayer.cpp

@@ -17,7 +17,7 @@ namespace pcpp
 		size_t val = 0;
 		std::string field = getCommandString();
 
-		for (size_t idx = 0; idx < field.size(); ++idx)
+		for (size_t idx = 0; idx < std::min(field.size(), static_cast<size_t>(8)); ++idx)
 		{
 			val |= static_cast<size_t>(field.c_str()[idx]) << (idx * 8);
 		}