SPM: Fix compiler warning

If passing an address to the macro "UNI_LISI_INIT_HEAD(head)",
the GNUARM compiler reports an warning:
“warning: the address of 'x' will always evaluate as 'true'”.

This patch is to eliminate this warning.

Change-Id: If07b5ed45f9757a78dbb68e25f1334c0cc9ab7d3
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
diff --git a/secure_fw/spm/include/lists.h b/secure_fw/spm/include/lists.h
index c630ff7..c0962c5 100644
--- a/secure_fw/spm/include/lists.h
+++ b/secure_fw/spm/include/lists.h
@@ -64,7 +64,7 @@
 
 /* Initialize the head node */
 #define UNI_LISI_INIT_HEAD(head) do {             \
-    if (head) {                                   \
+    if ((head) != NULL) {                         \
         (head)->next = NULL;                      \
     }                                             \
 } while (0)