check if output lines of controldata are possible to split (#797)

Otherwise it fails with scary stacktrace

patroni/postgresql.py

@@ -1216,7 +1216,8 @@ class Postgresql(object):
                 if data:
                     data = data.decode('utf-8').splitlines()
                     # pg_controldata output depends on major verion. Some of parameters are prefixed by 'Current '
-                    result = {l.split(':')[0].replace('Current ', '', 1): l.split(':', 1)[1].strip() for l in data if l}
+                    result = {l.split(':')[0].replace('Current ', '', 1): l.split(':', 1)[1].strip() for l in data
+                              if l and ':' in l}
             except subprocess.CalledProcessError:
                 logger.exception("Error when calling pg_controldata")
         return result