pbx.c: Crash in handle_hint_change due to uninitialized values

pbx.c: Crash in handle_hint_change due to uninitialized values

handle_hint_change calls extension_presence_state_helper, which returns the
presence state subtype and message when a valid state is available. If the
state is invalid then those values are not filled. If they have not been
properly initialized to NULL then when ast_free is later called on them
Asterisk crashes.

This patch initializes the subtype and message to NULL.

ASTERISK-25706 #close
patches:
  0008-handle_hint_change-initialize-presence_state.patch
   submitted by Tzafrir Cohen (license 5035)

Change-Id: I2eb08c68951b327c42df0798de60484c3a225a50