Safely check COM port attributes (#37)

Bug fix to check if the port product exists. This caused issues when
plugging in other USB COM devices, such as Arduinos who didn't have the
port.product attribute.

Squashed commit of the following:

commit 6247987
Author: Elliot Berman <berman5@purdue.edu>
Date:   Wed May 31 21:20:34 2017 -0500

    Fix #36

Author: edjubuh

prosflasher/ports.py

@@ -13,7 +13,14 @@ def list_com_ports():
     """
     :return: Returns a list of valid serial ports that we believe are VEX Cortex Microcontrollers
     """
-    return [x for x in serial.tools.list_ports.comports() if x.vid is not None and (x.vid in USB_VID or 'vex' in x.product.lower())]
+    def is_valid_port(p):
+        """
+        Returns true if the port is has a VEX product on it by the following conditions:
+        The Vendor ID matches the expected VEX Vendor ID (which is a default one)
+        or VEX occurs in the product name (if it exists)
+        """
+        return p.vid is not None and (p.vid in USB_VID or (isinstance(p.product, str) and 'vex' in p.product.lower()))
+    return [p for p in serial.tools.list_ports.comports() if is_valid_port(p)]
 
 
 def create_serial(port, parity):