patch 8.1.0963: illegal memory access when using 'incsearch'

Problem:    Illegal memory access when using 'incsearch'.
Solution:   Reset highlight_match when changing text. (closes #4022)
diff --git a/src/misc1.c b/src/misc1.c
index 75f4ab1..65d5e9e 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -2854,6 +2854,11 @@
 	changed_int();
     }
     ++CHANGEDTICK(curbuf);
+
+#ifdef FEAT_SEARCH_EXTRA
+    // If a pattern is highlighted, the position may now be invalid.
+    highlight_match = FALSE;
+#endif
 }
 
 /*