patch 8.1.1303: not possible to hide a balloon

Problem:    Not possible to hide a balloon.
Solution:   Hide the balloon when balloon_show() is called with an empty
            string or list.  Add balloon_gettext().
diff --git a/src/beval.h b/src/beval.h
index 090c5fb..60cf1ab 100644
--- a/src/beval.h
+++ b/src/beval.h
@@ -75,7 +75,7 @@
 #ifdef FEAT_VARTABS
     int			*vts;		// vartabstop setting for this buffer
 #endif
-    char_u		*msg;
+    char_u		*msg;		// allocated: current text
 #ifdef FEAT_GUI_MSWIN
     void		*tofree;
 #endif