dsp: q6lsm: Address use after free for mmap handle

The global declared mmap_handle can be left dangling
for case when the handle is freed by the calling function.
Fix is to address this. Also add a check to make sure
the mmap_handle is accessed legally.

Change-Id: I367f8a41339aa0025b545b125ee820220efedeee
Signed-off-by: Soumya Managoli <quic_c_smanag@quicinc.com>