fix too many or too few labels error when train a classifier for both linux and windows

src/data.c

@@ -546,7 +546,7 @@ void fill_truth(char *path, char **labels, int k, float *truth)
     char *filename = "";
     for (i = strlen(path) - 1; i >= 0; --i) {
         if (path[i] == '/' || path[i] == '\\') {
-            filename = malloc(strlen(path + i) + 1);
+            filename = (char*)malloc(strlen(path + i) + 1);
             strcpy(filename, path + i + 1);
             break;
         }