Prevent NULL pointer dereference in x509_ocsp.c

Add a check at the beginning of the function
mbedtls_x509_ocsp_response_free() to ensure that the pointer passed to
is not NULL. This prevent a NULL pointer dereference that could lead to
crashes.

library/x509_ocsp.c

@@ -79,7 +79,9 @@ void mbedtls_x509_ocsp_response_free( mbedtls_x509_ocsp_response *resp )
 {
     mbedtls_x509_ocsp_single_response *cur, *next;
 
-    if( resp->resp_type.p == NULL )
+    if( resp == NULL )
+        return;
+    else if( resp->raw.p == NULL )
         goto exit;
 
     /* Free list of certificates */