FIX explicit initialization of normalizer

Author: arjoly

sklearn/tree/_tree.c

@@ -3346,7 +3346,7 @@ cdef class Tree:
         cdef Node* node = nodes
         cdef Node* end_node = node + self.node_count
 
-        cdef double normalizer
+        cdef double normalizer = 0.
 
         cdef np.ndarray[np.float64_t, ndim=1] importances
         importances = np.zeros((self.n_features,))