Prevent arithmetic overflow on bounds check

library/ssl_cli.c

diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 585750e..759a456 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -2066,7 +2066,7 @@
     len = (*p)[0] << 8 | (*p)[1];
     *p += 2;
 
-    if( (*p) + len > end )
+    if( (*p) > end - len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message "
                                     "(psk_identity_hint length)" ) );