Fix bad loop condition within get_devices()

get_devices() includes this loop:

     char buffer[256];

     ...snip...

     char *end = buffer;

     ...snip...

     while (end && *end != ':')
         end++;

The condition "end" within the while guard will always be true, given that
buffer is on the stack and thus will never be near the zero value.  It
should be *end instead, to check for the NUL terminator byte: this code
will likely crash if the string does not contain a colon character.

This appears to have been present in the initial commit of the code
(8d6ad99)

Found by Braňo Náter using the "cppcheck" static analyzer.

Author: davidmalcolm

python-ethtool/ethtool.c

@@ -129,7 +129,7 @@ static PyObject *get_devices(PyObject *self __unused, PyObject *args __unused)
 		if (fgets(buffer, 256, fd) == NULL)
 			break;
 		/* find colon */
-		while (end && *end != ':')
+		while (*end && *end != ':')
 			end++;
 		*end = 0; /* terminate where colon was */
 		while (*name == ' ')