Safe copy crashes on large files.

Tested on 4.18.3

When copying large files (I assume single file has to be larger than available RAM), process crash when it comes to verifying file checksum.

Reproduce steps:

1. Edit -> Preferences -> Advanced -> Checksum while copying [always]
2. Copy very large file (tens of gigs, bigger than your RAM + SWAP)
3. Wait for checksumming operation

Log from ~/.xsession-errors:

(Thunar:1800): GLib-ERROR **: 21:17:43.256: ../glib/gmem.c:136: failed to allocate 51512490320 bytes
(thunar:11759): GLib-ERROR **: 21:32:37.912: ../glib/gmem.c:136: failed to allocate 51512490320 bytes

By the way, I want to thank the whole team for safe-copy functionality.