tools/libxc: Adjust error handling in map_p2m_list() to fix CentOS 7 build

tools/libxc: Adjust error handling in map_p2m_list() to fix CentOS 7 build

The "goto err;" for malloc() error handling would cause the cleanup code
to use 'ptes' before it had been initialised, and causing a build
failure because of -Werror=maybe-uninitialised.

Use "goto err;" consistently for all error handling.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>