drivers: shs: fix null check before freeing slow start list

Checks the correct pointer for null before freeing the associated page for
slow start shared memory. Fixes the following:

[ 2982.239281] Unable to handle kernel paging request at virtual address ffffffbfadadadb4
[ 2982.239512] pc : __free_pages+0x24/0xc0
[ 2982.239515] lr : free_pages+0x38/0x48
[ 2982.240605] Call trace:
[ 2982.240609] __free_pages+0x24/0xc0
[ 2982.240613] free_pages+0x38/0x48
[ 2982.240632] rmnet_shs_release_ss_flows+0x38/0x58 [rmnet_shs]

Change-Id: I1c61b8c9c89905e94c24f6836eaf1d7f56566162
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>