Apply a default baudrate when the OS does not provide one.

Handle the situation when the OS does not provide a baudrate in the code
path which opens the serial port. See bug #1632 and PR #6 for details.

  https://sigrok.org/bugzilla/show_bug.cgi?id=1632
  https://github.com/sigrokproject/libserialport/pull/6

This resolves bug #1632.

[ gsi: adjust comment style, rephrase comment and commit message ]

serialport.c

@@ -590,6 +590,15 @@ SP_API enum sp_return sp_open(struct sp_port *port, enum sp_mode flags)
 		RETURN_CODEVAL(ret);
 	}
 
+	/*
+	 * Assume a default baudrate if the OS does not provide one.
+	 * Cannot assign -1 here since Windows holds the baudrate in
+	 * the DCB and does not configure the rate individually.
+	 */
+	if (config.baudrate == 0) {
+		config.baudrate = 9600;
+	}
+
 	/* Set sane port settings. */
 #ifdef _WIN32
 	data.dcb.fBinary = TRUE;