Fix memory over-read

Fix from David Baggett via tweet.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>

crypto/bn/bn_lib.c

diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index c8e8519..2ca6bea 100644
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -553,7 +553,7 @@
         return (NULL);
     bn_check_top(ret);
     /* Skip leading zero's. */
-    for ( ; *s == 0 && len > 0; s++, len--)
+    for ( ; len > 0 && *s == 0; s++, len--)
         continue;
     n = len;
     if (n == 0) {