When calling an empty macro, do not clobber existing arguments.

Fixing a bug found with the groffer(1) version 1.19 manual page
following a report from Jan Stary.

3 files changed, 30 insertions, 3 deletions

diff --git a/regress/usr.bin/mandoc/roff/de/Makefile b/regress/usr.bin/mandoc/roff/de/Makefile
index 6760dc6e21a..86ea8c8c56c 100644
--- a/regress/usr.bin/mandoc/roff/de/Makefile
+++ b/regress/usr.bin/mandoc/roff/de/Makefile
@@ -1,7 +1,7 @@
-# $OpenBSD: Makefile,v 1.12 2019/02/06 20:54:28 schwarze Exp $
+# $OpenBSD: Makefile,v 1.13 2019/04/21 23:45:50 schwarze Exp $
 
-REGRESS_TARGETS	 = append cond escname factorial indir infinite startde tab
-REGRESS_TARGETS	+= TH Dd
+REGRESS_TARGETS	 = append cond empty escname factorial
+REGRESS_TARGETS	+= indir infinite startde tab TH Dd
 LINT_TARGETS	 = escname indir infinite
 
 # groff-1.22.4 defect:
diff --git a/regress/usr.bin/mandoc/roff/de/empty.in b/regress/usr.bin/mandoc/roff/de/empty.in
new file mode 100644
index 00000000000..decfcb51077
--- /dev/null
+++ b/regress/usr.bin/mandoc/roff/de/empty.in
@@ -0,0 +1,18 @@
+.\" $OpenBSD: empty.in,v 1.1 2019/04/21 23:45:50 schwarze Exp $
+.Dd $Mdocdate: April 21 2019 $
+.Dt DE-EMPTY 1
+.Os
+.Sh NAME
+.Nm de-empty
+.Nd empty user-defined macro with arguments
+.Sh DESCRIPTION
+initial text
+.de empty
+..
+.de real
+arg=\\$1
+.empty wrong
+arg=\\$1
+..
+.real right
+final text
diff --git a/regress/usr.bin/mandoc/roff/de/empty.out_ascii b/regress/usr.bin/mandoc/roff/de/empty.out_ascii
new file mode 100644
index 00000000000..1883fd3e98e
--- /dev/null
+++ b/regress/usr.bin/mandoc/roff/de/empty.out_ascii
@@ -0,0 +1,9 @@
+DE-EMPTY(1)                 General Commands Manual                DE-EMPTY(1)
+
+NNAAMMEE
+     ddee--eemmppttyy - empty user-defined macro with arguments
+
+DDEESSCCRRIIPPTTIIOONN
+     initial text arg=right arg=right final text
+
+OpenBSD                         April 21, 2019                         OpenBSD