Don't perform arithmetic on NULL pointers

When node is NULL when calling getparent() et al. they return NULL
but performing arithmetic on the NULL pointer is undefined.  Check
if 'node' or 'header' is NULL and skip the adjustment.

lib/dns/rbt.c

@@ -127,7 +127,7 @@ struct file_header {
 					  */
 	unsigned int nodecount;		 /* shadow from rbt structure */
 	uint64_t crc;
-	char version2[32]; /* repeated; must match version1 */
+	char version2[32];		 /* repeated; must match version1 */
 };
 
 /*
@@ -168,6 +168,10 @@ serialize_nodes(FILE *file, dns_rbtnode_t *node, uintptr_t parent,
 		dns_rbtdatawriter_t datawriter, void *writer_arg,
 		uintptr_t *where, uint64_t *crc);
 
+#define ADJUST_ADDRESS(address, which, header)              \
+	if (address != NULL && header != NULL) {            \
+		address += node->which * (uintptr_t)header; \
+	}
 /*
  * The following functions allow you to get the actual address of a pointer
  * without having to use an if statement to check to see if that address is
@@ -176,7 +180,8 @@ serialize_nodes(FILE *file, dns_rbtnode_t *node, uintptr_t parent,
 static dns_rbtnode_t *
 getparent(dns_rbtnode_t *node, file_header_t *header) {
 	char *adjusted_address = (char *)(node->parent);
-	adjusted_address += node->parent_is_relative * (uintptr_t)header;
+
+	ADJUST_ADDRESS(adjusted_address, parent_is_relative, header);
 
 	return ((dns_rbtnode_t *)adjusted_address);
 }
@@ -184,7 +189,8 @@ getparent(dns_rbtnode_t *node, file_header_t *header) {
 static dns_rbtnode_t *
 getleft(dns_rbtnode_t *node, file_header_t *header) {
 	char *adjusted_address = (char *)(node->left);
-	adjusted_address += node->left_is_relative * (uintptr_t)header;
+
+	ADJUST_ADDRESS(adjusted_address, left_is_relative, header);
 
 	return ((dns_rbtnode_t *)adjusted_address);
 }
@@ -192,7 +198,8 @@ getleft(dns_rbtnode_t *node, file_header_t *header) {
 static dns_rbtnode_t *
 getright(dns_rbtnode_t *node, file_header_t *header) {
 	char *adjusted_address = (char *)(node->right);
-	adjusted_address += node->right_is_relative * (uintptr_t)header;
+
+	ADJUST_ADDRESS(adjusted_address, right_is_relative, header);
 
 	return ((dns_rbtnode_t *)adjusted_address);
 }
@@ -200,7 +207,8 @@ getright(dns_rbtnode_t *node, file_header_t *header) {
 static dns_rbtnode_t *
 getdown(dns_rbtnode_t *node, file_header_t *header) {
 	char *adjusted_address = (char *)(node->down);
-	adjusted_address += node->down_is_relative * (uintptr_t)header;
+
+	ADJUST_ADDRESS(adjusted_address, down_is_relative, header);
 
 	return ((dns_rbtnode_t *)adjusted_address);
 }
@@ -208,7 +216,8 @@ getdown(dns_rbtnode_t *node, file_header_t *header) {
 static dns_rbtnode_t *
 getdata(dns_rbtnode_t *node, file_header_t *header) {
 	char *adjusted_address = (char *)(node->data);
-	adjusted_address += node->data_is_relative * (uintptr_t)header;
+
+	ADJUST_ADDRESS(adjusted_address, data_is_relative, header);
 
 	return ((dns_rbtnode_t *)adjusted_address);
 }