winansi: avoid buffer overrun

When we could not convert the UTF-8 sequence into Unicode for writing to
the Console, we should not try to write an insanely-long sequence of
invalid wide characters (mistaking the negative return value for an
unsigned length).

Reported by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/compat/winansi.c b/compat/winansi.c
index a551de9..a11a0f1 100644
--- a/compat/winansi.c
+++ b/compat/winansi.c
@@ -140,6 +140,11 @@ static void write_console(unsigned char *str, size_t len)
 
 	/* convert utf-8 to utf-16 */
 	int wlen = xutftowcsn(wbuf, (char*) str, ARRAY_SIZE(wbuf), len);
+	if (wlen < 0) {
+		wchar_t *err = L"[invalid]";
+		WriteConsoleW(console, err, wcslen(err), &dummy, NULL);
+		return;
+	}
 
 	/* write directly to console */
 	WriteConsoleW(console, wbuf, wlen, &dummy, NULL);