Sanity check input length in OPENSSL_uni2asc().

Thanks to Hanno Böck for reporting this bug.
Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/pkcs12/p12_utl.c

@@ -42,7 +42,9 @@ char *OPENSSL_uni2asc(unsigned char *uni, int unilen)
 {
     int asclen, i;
     char *asctmp;
-
+    /* string must contain an even number of bytes */
+    if (unilen & 1)
+        return NULL;
     asclen = unilen / 2;
     /* If no terminating zero allow for one */
     if (!unilen || uni[unilen - 1])