xkbcomp: Don't falsely promise from ExprResolveLhs

Every user of ExprReturnLhs goes on to unconditionally dereference the
field return, which can be NULL if xkb_intern_atom fails. Return false
if this is the case, so we fail safely.

testcase: splice geometry data into interp

Signed-off-by: Daniel Stone <daniels@collabora.com>