pbx: Prevent Realtime switch crash on invalid priority

pbx_extension_helper takes two 'context' arguments. One (con) is a
pointer directly to a 'struct ast_context' and the other (context) is
the name of the context. In all cases, one of these arguments is NULL
and the other is non-NULL.

Functions that are ultimately called by pbx_extension_helper expect that
'context' will be non-NULL, so we set it unconditionally on entry into
this function.

ASTERISK-28534 #close

Change-Id: Ifbbc5e71440afd80efd441f7a9d72e8b10b6f47d