patch 8.1.2236: ml_get error if pattern matches beyond last line

Problem:    Ml_get error if pattern matches beyond last line.
Solution:   Adjust position if needed. (Christian Brabandt, closes #5139)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index fc692e3..a7f80a8 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3947,6 +3947,11 @@
 		    VIM_CLEAR(sub_firstline);
 		}
 
+		// Match might be after the last line for "\n\zs" matching at
+		// the end of the last line.
+		if (lnum > curbuf->b_ml.ml_line_count)
+		    break;
+
 		if (sub_firstline == NULL)
 		{
 		    sub_firstline = vim_strsave(ml_get(sub_firstlnum));