imDefLkup: verify that a pointer isn't NULL before using it

It is possible for _XimICOfXICID() to return NULL, so it is necessary
to check this isn't actually the case before dereferencing the pointer.
All other callers of _XimICOfXICID() do this check too.

(The check itself is ugly, but it follows the style of the code in the
rest of the module.)

Fixes issue #45.

Reported-by: Bhavi Dhingra

Original-patch-by: Bhavi Dhingra

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>
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