patch 8.2.1633: some error messages are internal but do not use iemsg()

Problem:    Some error messages are internal but do not use iemsg().
Solution:   Use iemsg(). (Dominique Pellé, closes #6894)
diff --git a/src/regexp.c b/src/regexp.c
index 22b2d17..f7f04ea 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -2279,7 +2279,7 @@
 		    else if (*s == NUL) // we hit NUL.
 		    {
 			if (copy)
-			    emsg(_(e_re_damg));
+			    iemsg(_(e_re_damg));
 			goto exit;
 		    }
 		    else