rtu layer: fix parsing of serial stop bits values

Up until now, the stop bits value was hard coded to be 2 if the parity
mode was set to none or 1 if the parity mode was set to even or odd.

So a setting of "115200E1" would result in a stop bit value of 1.
But "115200E2" would result also in a stop bit value of 1.
And "115200N1" would result in a stop bit value of 2.

This is not a restriction of libmodbus. The library supports all these
modes just fine. In fact they are using 115200N1 in the documentation
for modbus_new_rtu().

Rather this is a bug in the string parsing that libmodbuspp introduces
in its constructor.
libmodbus has no string parsing in modbus_new_rtu() and takes in the
arguments as parameters directly.

Fixes: b0f3dc4

Author: tamara-schmitz

src/rtulayer.cpp

@@ -15,6 +15,7 @@
  * along with the libmodbuspp Library; if not, see <http://www.gnu.org/licenses/>.
  */
 #include <modbuspp/message.h>
+#include <iostream>
 #include "rtulayer_p.h"
 #include "config.h"
 
@@ -239,6 +240,7 @@ namespace Modbus {
       b = std::stoi (settings);
     }
     catch (...) {
+      std::cerr << "ERROR in parsing RtuLayer settings string \"" << settings << "\": Invalid baud. Setting to 19200. " << std::endl;
       b = 19200;
     }
     return b;
@@ -256,17 +258,29 @@ namespace Modbus {
         return c;
       }
     }
+
+    std::cerr << "ERROR in parsing RtuLayer settings string \"" << settings << "\": Invalid parity. Setting to N. " << std::endl;
+
     return p;
   }
 
   // ---------------------------------------------------------------------------
   // static
   int RtuLayer::stop (const std::string & settings) {
+    size_t s = settings.length();
 
-    if (parity (settings) == 'N') {
-
-      return 2;
+    if (s >= 3) {
+      char c = settings[s - 1];
+      switch (c) {
+        case '1':
+          return 1;
+        case '2':
+          return 2;
+      }
     }
+
+    std::cerr << "ERROR in parsing RtuLayer settings string \"" << settings << "\": Invalid stop bit. Setting to 1. " << std::endl;
+
     return 1;
   }