check the return value of dec64(2)

Reference: /n/atom/patch/applied2013/imap4ddec64
Date: Wed Jun 19 00:17:25 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/imap4d/auth.c	Wed Jun 19 00:17:17 2013
+++ /sys/src/cmd/upas/imap4d/auth.c	Wed Jun 19 00:17:17 2013
@@ -140,8 +140,6 @@
 		return nil;
 	s = binalloc(&parsebin, n + 1, 1);
 	n = dec64((uchar*)s, n, t, n);
-	if(n == -1)
-		return nil;
 	s[n] = 0;
 	return s;
 }
@@ -275,8 +273,6 @@
 	if(ch == nil || strlen(ch) == 0)
 		return Ecancel;
 	n  = dec64((uchar*)buf, sizeof buf, ch, strlen(ch));
-	if(n == -1)
-		return Ebadau;
 	nf = niltokenize(buf, n, f, nelem(f));
 	if(nf != 3)
 		return Ebadau;