Fix rb-tree init (#349)

The struct member right_red is used in several functions, such as 
rb_node_set_right(). It has been reported by infer and LLVM static 
analyzer that right_red isn't initialized before used.

By tracing the node initialization function calls (starting from 
map_create_node), it can be seen that rb_node_set_right is the function
where right_red is attempted to be initialized, but we are indeed 
performing &1 on an uninitialized value. 

In this commit, a change to using calloc guarantees the struct members 
will be zeroed out during allocation, which in terms serves as 
initialization.

src/map.c

@@ -548,7 +548,7 @@ static map_node_t *map_create_node(void *key,
                                    size_t ksize,
                                    size_t vsize)
 {
-    map_node_t *node = malloc(sizeof(map_node_t));
+    map_node_t *node = calloc(1, sizeof(map_node_t));
     assert(node);
 
     /* allocate memory for the keys and data */