mtd: ts5500_flash: avoid calling map_destroy on NULL

map_destroy dereferences its argument.  The call is furthermore only
reachable when this argument is NULL.  Thus the call is dropped.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression *x;
@@

*if (x == NULL)
{ ...
* map_destroy(x);
  ...
  return ...;
}
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>