patch 8.0.1517: invalid memory acces with pattern using look-behind match

Problem:    Invalid memory acces with pattern using look-behind match.
            (Dominique Pelle)
Solution:   Get a pointer to the right line.
diff --git a/src/regexp.c b/src/regexp.c
index d07391e..b1cad99 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -5585,9 +5585,14 @@
 		    {
 #ifdef FEAT_MBYTE
 			if (has_mbyte)
+			{
+			    char_u *line =
+					 reg_getline(behind_pos.rs_u.pos.lnum);
+
 			    rp->rs_un.regsave.rs_u.pos.col -=
-				(*mb_head_off)(regline, regline
+				(*mb_head_off)(line, line
 				    + rp->rs_un.regsave.rs_u.pos.col - 1) + 1;
+			}
 			else
 #endif
 			    --rp->rs_un.regsave.rs_u.pos.col;