lineedit: fix completion with Unicode chars

function                                             old     new   delta
read_line_input                                     4966    5002     +36
bb_wcstombs                                          170     159     -11
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 36/-11)             Total: 25 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/libbb/unicode.c b/libbb/unicode.c
index c4b5f86..70c6abe 100644
--- a/libbb/unicode.c
+++ b/libbb/unicode.c
@@ -131,7 +131,7 @@
 		size_t len = wcrtomb_internal(tbuf, wc);
 
 		if (len > n)
-			len = n;
+			break;
 		memcpy(dest, tbuf, len);
 		if (wc == L'\0')
 			return org_n - n;