mysql_user: reinitialize the privs list in privileges_unpack()

In some scenarios, `privileges_unpack()` called `privs.append()` inside a loop
without first emptying or reinitializing the `privs` list from the prior
iteration. This could result in an invalid `GRANT` statement, which incorrectly
included privileges from a previously-built `GRANT` statement.

Reinitialize `privs` on each pass of the loop to prevent this from occurring.

plugins/modules/mysql_user.py

@@ -882,8 +882,8 @@ def privileges_unpack(priv, mode):
     else:
         quote = '`'
     output = {}
-    privs = []
     for item in priv.strip().split('/'):
+        privs = []
         pieces = item.strip().rsplit(':', 1)
         dbpriv = pieces[0].rsplit(".", 1)