uart_ow.hpp: Correct a logic bug, and 2 questionable uses of c_str()

The command() function had a logic error that caused a command to be
broadcast to all devices on the DS OW bus when a proper ID was
specified.  This should only be done when NO ID is specified.

This fixes UPM issue #502.

In addition, there were two cases where the C++ string accessor method
.c_str() was used when .data() should be used instead.  This worked
anyway (accidentally), but is improper since the strings can have
embedded 0 bytes.

Signed-off-by: Jon Trulson <jtrulson@ics.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/uart_ow.hpp

@@ -233,15 +233,15 @@ class UartOW
     mraa::Result
     command(uint8_t command, std::string id)
     {
-        if (id.empty() == 0)
+        if (id.empty())
             return (mraa::Result) mraa_uart_ow_command(m_uart, command, NULL);
         else {
             if (id.size() != 8) {
                 // Only 8 byte romcodes are legal.
                 throw std::invalid_argument(std::string(__FUNCTION__) +
                                             ": id must be 8 bytes only");
             }
-            return (mraa::Result) mraa_uart_ow_command(m_uart, command, (uint8_t*) id.c_str());
+            return (mraa::Result) mraa_uart_ow_command(m_uart, command, (uint8_t*) id.data());
         }
     }
 
@@ -268,7 +268,7 @@ class UartOW
     uint8_t
     crc8(std::string buffer)
     {
-        return mraa_uart_ow_crc8((uint8_t*) buffer.c_str(), buffer.size());
+        return mraa_uart_ow_crc8((uint8_t*) buffer.data(), buffer.size());
     }
 
   private: