Add NULL pointer check for consistency

Most other functions in this module have a similar check.

library/pk.c

diff --git a/library/pk.c b/library/pk.c
index c793fd8..ba8950e 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -206,8 +206,8 @@
 static int pk_restart_setup( mbedtls_pk_restart_ctx *ctx,
                                const mbedtls_pk_info_t *info )
 {
-    /* Don't do anything it already set up */
-    if( ctx->pk_info != NULL )
+    /* Don't do anything if already set up or invalid */
+    if( ctx == NULL || ctx->pk_info != NULL )
         return( 0 );
 
     /* Should never happen when we're called */