Fix some potential UB in tectonic_xdv, and cargo update

[pkgw]

As reported in #752.

[codecov]

Codecov Report

Merging #753 (8a86099) into master (28a9b46) will decrease coverage by 0.00%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##           master     #753      +/-   ##
==========================================
- Coverage   46.80%   46.80%   -0.01%     
==========================================
  Files         118      118              
  Lines       53707    53705       -2     
==========================================
- Hits        25139    25138       -1     
+ Misses      28568    28567       -1     

Impacted Files	Coverage Δ
src/document.rs	69.23% <57.14%> (+0.16%)	⬆️
src/io/memory.rs	81.53% <71.42%> (ø)

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