Fix warning compiling with Clang 7 on ARM64:

prefix_tree.c:158:8: error: result of comparison of constant -1 with expression of type 'char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]

modules/drouting/prefix_tree.c

@@ -34,22 +34,23 @@ extern int inode;
 extern int unode;
 
 #define DR_PREFIX_ARRAY_SIZE 128
-static unsigned char *dr_char2idx = NULL;
+static signed char *dr_char2idx = NULL;
 
 /* number of children under a prefix node */
 int ptree_children = 0;
 
 #define IDX_OF_CHAR(_c) \
 	dr_char2idx[ (unsigned char)(_c) ]
+#define UIDX_OF_CHAR(_c) (unsigned char)IDX_OF_CHAR(_c)
 
 #define IS_VALID_PREFIX_CHAR(_c) \
-	((((unsigned char)(_c))<DR_PREFIX_ARRAY_SIZE) && (char)IDX_OF_CHAR(_c)!=-1 )
+	((((unsigned char)(_c))<DR_PREFIX_ARRAY_SIZE) && IDX_OF_CHAR(_c)!=-1 )
 
 int init_prefix_tree( char *extra_prefix_chars )
 {
 	int i;
 
-	dr_char2idx = (unsigned char *)pkg_malloc
+	dr_char2idx = (signed char *)pkg_malloc
 		( DR_PREFIX_ARRAY_SIZE * sizeof(unsigned char) );
 	if (dr_char2idx==NULL) {
 		LM_ERR("not enought pkg mem for the prefix array\n");
@@ -271,9 +272,9 @@ add_prefix(
 		if( tmp == (prefix->s+prefix->len-1) ) {
 			/* last digit in the prefix string */
 			LM_DBG("adding info %p, %d at: "
-				"%p (%d)\n", r, rg, &(ptree->ptnode[IDX_OF_CHAR(*tmp)]),
+				"%p (%d)\n", r, rg, &(ptree->ptnode[UIDX_OF_CHAR(*tmp)]),
 				IDX_OF_CHAR(*tmp));
-			res = add_rt_info(&(ptree->ptnode[IDX_OF_CHAR(*tmp)]),
+			res = add_rt_info(&(ptree->ptnode[UIDX_OF_CHAR(*tmp)]),
 					r,rg, malloc_f, free_f);
 			if(res < 0 ) {
                 LM_ERR("adding rt info doesn't work\n");
@@ -284,13 +285,13 @@ add_prefix(
 			goto ok_exit;
 		}
 		/* process the current digit in the prefix */
-		if(NULL == ptree->ptnode[IDX_OF_CHAR(*tmp)].next) {
+		if(NULL == ptree->ptnode[UIDX_OF_CHAR(*tmp)].next) {
 			/* allocate new node */
 			INIT_PTREE_NODE(malloc_f, ptree,
-				ptree->ptnode[IDX_OF_CHAR(*tmp)].next);
+				ptree->ptnode[UIDX_OF_CHAR(*tmp)].next);
 			inode+=10;
 		}
-		ptree = ptree->ptnode[IDX_OF_CHAR(*tmp)].next;
+		ptree = ptree->ptnode[UIDX_OF_CHAR(*tmp)].next;
 		tmp++;
 	}