loader.efi: find_currdev() can leak memory

The find_currdev() is using variable "copy" to store the reference to trimmed
devpath pointer, if for some reason the efi_devpath_handle() fails, we will
leak this copy.

Also we can simplify the code there a bit.

Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D9191