migration: Remove unneeded NULL check from migrate_fd_error()

All the callers of migrate_fd_error() pass a non-NULL
error parameter, and if any did pass NULL then we would
segfault in error_copy(), so remove the unnecessary
NULL check earlier in the function.
(Spotted by Coverity.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

migration/migration.c

@@ -922,7 +922,7 @@ static void migrate_fd_cleanup(void *opaque)
 
 void migrate_fd_error(MigrationState *s, const Error *error)
 {
-    trace_migrate_fd_error(error ? error_get_pretty(error) : "");
+    trace_migrate_fd_error(error_get_pretty(error));
     assert(s->to_dst_file == NULL);
     migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
                       MIGRATION_STATUS_FAILED);