[efi] Avoid modifying PE/COFF debug filename

[efi] Avoid modifying PE/COFF debug filename

The function efi_pecoff_debug_name() (called by efi_handle_name()) is
used to extract a filename from the debug data directory entry located
within a PE/COFF image.  The name is copied into a temporary static
buffer to allow for modifications, but the code currently erroneously
modifies the original name within the loaded PE/COFF image.

Fix by performing the modification on the copy in the temporary
buffer, as originally intended.

Signed-off-by: Michael Brown <mcb30@ipxe.org>