patch 7.4.821
Problem:    Coverity reports a few problems.
Solution:   Avoid the warnings. (Christian Brabandt)
diff --git a/src/screen.c b/src/screen.c
index ba964b4..b1af862 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -7801,7 +7801,7 @@
 	}
     }
     posmatch->cur = 0;
-    if (shl->lnum == lnum)
+    if (shl->lnum == lnum && bot >= 0)
     {
 	colnr_T	start = posmatch->pos[bot].col == 0
 					     ? 0 : posmatch->pos[bot].col - 1;