efi_loader: pkcs7_parse_message() returns error pointer

Since pkcs7_parse_message() returns an error pointer, we must not
check for NULL.  We have to explicitly set msg to NULL in the error
case, otherwise the call to pkcs7_free_message() on the goto err
path will assume it's a valid object.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Add missing include linux/err.h
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>