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>

drivers/nfc/nfcsim.c

@@ -482,8 +482,10 @@ static int __init nfcsim_init(void)
 exit_err:
 	pr_err("Failed to initialize nfcsim driver (%d)\n", rc);
 
-	nfcsim_link_free(link0);
-	nfcsim_link_free(link1);
+	if (link0)
+		nfcsim_link_free(link0);
+	if (link1)
+		nfcsim_link_free(link1);
 
 	return rc;
 }