NFC: add NULL checks to avoid potential NULL pointer dereference

NULL checks at line 457: if (!link0 || !link1) {, implies that both
pointers link0 and link1 might be NULL.
Function nfcsim_link_free() dereference pointers link0 and link1.
Add NULL checks before calling nfcsim_link_free() to avoid a
potential NULL pointer dereference.

Addresses-Coverity-ID: 1364857
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>