patch 8.1.2107: various memory leaks reported by asan

Problem:    Various memory leaks reported by asan.
Solution:   Free the memory. (Ozaki Kiichi, closes #5003)
diff --git a/src/popupwin.c b/src/popupwin.c
index d4b3d7c..a77d98b 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -3365,6 +3365,7 @@
 	    trunc_string(wp->w_popup_title, title, total_width - 2, len);
 	    screen_puts(title, wp->w_winrow, wp->w_wincol + 1,
 		    wp->w_popup_border[0] > 0 ? border_attr[0] : popup_attr);
+	    vim_free(title);
 	}
 
 	// Compute scrollbar thumb position and size.