check syntax of private key (#1568)

Check that header and footer of the private key are present in order to detect
corrupted key files.

Signed-off-by: Sascha Silbe <sascha-pgp@silbe.org>

src/sugar/profile.py

@@ -105,14 +105,18 @@ class Profile(object):
             return None
 
         key = ""
+        begin_found = False
+        end_found = False
         for l in lines:
             l = l.strip()
             if l.startswith("-----BEGIN DSA PRIVATE KEY-----"):
+                begin_found = True
                 continue
             if l.startswith("-----END DSA PRIVATE KEY-----"):
+                end_found = True
                 continue
             key += l
-        if not len(key):
+        if not (len(key) and begin_found and end_found):
             logging.error("Error parsing public key.")
             return None