Fix possible method call on None object (CWE-476)

If we don't have current device parsed yet, doesn't make sense to
parse next lines of ifconfig output.

Author: frenzymadness

tests/parse_ifconfig.py

@@ -113,6 +113,11 @@ def __init__(self, stdout=None, debug=False):
                 curdev._parse_rest_of_first_line_old(mo.group(2))
                 continue
 
+            # If we don't have current device yet, doesn't make sense to
+            # read the rest of output lines
+            if curdev is None:
+                continue
+
             if self.oldFormat:
                 curdev._parse_line_old(line)
             else: