Fix bug in server parsing point formats extension

This bug becomes noticeable when the extension following the "supported point
formats" extension has a number starting with 0x01, which is the case of the
EC J-PAKE extension, which explains what I noticed the bug now.

This will be immediately backported to the stable branches,
see the corresponding commits for impact analysis.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 67dad5f..aab25e2 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -299,7 +299,7 @@
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
-    p = buf + 2;
+    p = buf + 1;
     while( list_size > 0 )
     {
         if( p[0] == MBEDTLS_ECP_PF_UNCOMPRESSED ||