ata: libata-core: fix NULL pointer deref in ata_host_alloc_pinfo()

In the Linux kernel, the following vulnerability has been resolved:

ata: libata-core: fix NULL pointer deref in ata_host_alloc_pinfo()

In an unlikely (and probably wrong?) case that the 'ppi' parameter of

ata_host_alloc_pinfo() points to an array starting with a NULL pointer,

there's going to be a kernel oops as the 'pi' local variable won't get

reassigned from the initial value of NULL. Initialize 'pi' instead to

'&ata_dummy_port_info' to fix the possible kernel oops for good...

Found by Linux Verification Center (linuxtesting.org) with the SVACE static

analysis tool.