selection: reset ‘empty_count’ after we’ve emitted the empty cells

When marking and unmarking cells, we don’t highlight trailing empty
cells. We do however highlight empty cells if they are followed by
non-empty cells.

I think this was an intentional choice. If one row ended with trailing
empty cells, but *no* hard linebreak, then we’d continue on the next
row, and emit all the empty cells once we hit a non-emtpy cell on the
second row.

But this is something that shouldn’t happen in any real-world use
cases.

selection.c

@@ -489,6 +489,7 @@ premark_selected(struct terminal *term, struct row *row, struct cell *cell,
     for (int i = 0; i < ctx->empty_count + 1; i++)
         row->cells[col - i].attrs.selected |= 2;
 
+    ctx->empty_count = 0;
     return true;
 }
 
@@ -520,6 +521,7 @@ mark_selected(struct terminal *term, struct row *row, struct cell *cell,
         }
     }
 
+    ctx->empty_count = 0;
     return true;
 }