supervisor: Always allocate at least a 1x1 terminal

Otherwise, out of range writes would occur in tilegrid_set_tile, causing a safe mode reset.
```
Hardware watchpoint 6: -location *stack_alloc->ptr

Old value = 24652061
New value = 24641565
0x000444f2 in common_hal_displayio_tilegrid_set_tile (self=0x200002c8 <supervisor_terminal_text_grid>, x=1, y=1, tile_index=0 '\000')
    at ../../shared-module/displayio/TileGrid.c:236
236	    if (!self->partial_change) {
(gdb)
```

Author: jepler

supervisor/shared/display.c

@@ -65,10 +65,14 @@ void supervisor_start_terminal(uint16_t width_px, uint16_t height_px) {
     if (width_in_tiles < 80) {
         scale = 1;
     }
+
     width_in_tiles = (width_px - blinka_bitmap.width * scale) / (grid->tile_width * scale);
+    if (width_in_tiles < 1) {
+        width_in_tiles = 1;
+    }
     uint16_t height_in_tiles = height_px / (grid->tile_height * scale);
     uint16_t remaining_pixels = height_px % (grid->tile_height * scale);
-    if (remaining_pixels > 0) {
+    if (height_in_tiles < 1 || remaining_pixels > 0) {
         height_in_tiles += 1;
     }
 
@@ -94,6 +98,8 @@ void supervisor_start_terminal(uint16_t width_px, uint16_t height_px) {
     }
     grid->width_in_tiles = width_in_tiles;
     grid->height_in_tiles = height_in_tiles;
+    assert(width_in_tiles > 0);
+    assert(height_in_tiles > 0);
     grid->pixel_width = width_in_tiles * grid->tile_width;
     grid->pixel_height = height_in_tiles * grid->tile_height;
     grid->tiles = tiles;