efi_loader: loosen buffer parameter check in efi_file_read

efi_loader: loosen buffer parameter check in efi_file_read

When reading a directory, EFI_BUFFER_TOO_SMALL should be returned when
the supplied buffer is too small, so a use-case is to call
efi_file_read with *buffer_size=0 and buffer=NULL to obtain the needed
size before doing the actual read.

So move the buffer!=NULL check to after the buffer size has been checked.

This fix allows the Redhat shim fallback to run and e.g. Fedora 32 now
boots out of the box.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>