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>
