pcm: dshare - fix shared memory pointer check

pcm: dshare - fix shared memory pointer check

Currently shared memory pointer is initialized to 0 and set to -1
in some, but not in all error paths.
In cleanup path of open the shm pointer is only compared to be non-NULL
before dereferencing it which leads to SEGFAULT in case it was set to -1.

This patch initializes pointer to -1 to have a unique identification
for invalid pointer and also checks for pointer being not -1 on
access in cleanup path.

Signed-off-by: Vanitha Channaiah <vanitha.channaiah@in.bosch.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>