migration: fix maybe-uninitialized warning

migration: fix maybe-uninitialized warning

../migration/ram.c: In function ‘multifd_recv_thread’:
/home/elmarco/src/qq/include/qapi/error.h:165:5: error: ‘block’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  165 |     error_setg_internal((errp), __FILE__, __LINE__, __func__,   \
      |     ^~~~~~~~~~~~~~~~~~~
../migration/ram.c:818:15: note: ‘block’ was declared here
  818 |     RAMBlock *block;
      |               ^~~~~

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>