Fix overtyping wide characters.

Overtyping the first half of a wide character with the
second half of a wide character results in display garbage.
This is because the trailing dummy is not cleaned up.

i.e.  ATTR_WIDE, ATTR_WDUMMY, ATTR_WDUMMY

Here is a short script for demonstrating the behavior:

	#!/bin/sh
	alias printf=/usr/bin/printf
	printf こんにちは！; sleep 2
	printf '\x1b[5D'; sleep 2
	printf へ; sleep 2
	printf ' '; sleep 2
	echo

1 files changed, 4 insertions, 0 deletions

diff --git a/st.c b/st.c
index de2dd0e..51049ba 100644
--- a/st.c
+++ b/st.c
@@ -2507,6 +2507,10 @@ check_control_code:
         if (width == 2) {
                 gp->mode |= ATTR_WIDE;
                 if (term.c.x+1 < term.col) {
+                        if (gp[1].mode == ATTR_WIDE && term.c.x+2 < term.col) {
+                                gp[2].u = ' ';
+                                gp[2].mode &= ~ATTR_WDUMMY;
+                        }
                         gp[1].u = '\0';
                         gp[1].mode = ATTR_WDUMMY;
                 }