w_grid_alloc: baseline impl

src/nvim/highlight.c

@@ -517,6 +517,10 @@ static HlAttrs get_colors_force(int attr)
 /// @return the resulting attributes.
 int hl_blend_attrs(int back_attr, int front_attr, bool *through)
 {
+  if (front_attr < 0 || back_attr < 0) {
+    return -1;
+  }
+
   HlAttrs fattrs = get_colors_force(front_attr);
   int ratio = fattrs.hl_blend;
   if (ratio <= 0) {