patch 8.2.2244: crash when making the window width negative

Problem:    Crash when making the window width of the not-current window
            negative.
Solution:   Make sure the window width is not negative. (closes #7568)

src/window.c

@@ -5687,6 +5687,8 @@ win_setwidth_win(int width, win_T *wp)
 	if (width == 0)
 	    width = 1;
     }
+    else if (width < 0)
+	width = 0;
 
     frame_setwidth(wp->w_frame, width + wp->w_vsep_width);