mailinfo: feed the correct line length to decode_transfer_encoding()

When handling a MIME multipart message, multi-part boundary lines are eaten
by a call to handle_boundary() function from the main loop of handle_body(),
and after that happens, we should update the line length correctly, because
handle_boundary() udpates line[] with new data.

This was caused by a thinko in 9aa23094 (mailinfo: apply the same fix not
to lose NULs in BASE64 and QP codepaths, 2008-05-25).
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-mailinfo.c

@@ -812,6 +812,7 @@ static void handle_body(void)
 					      np - newline);
 			if (!handle_boundary())
 				return;
+			len = strlen(line);
 		}
 
 		/* Unwrap transfer encoding */