Fixed deadlock in Library::writeBookmarksToFile()

[veloman-yunkan]

Fixes #641

No unit was added.

[codecov]

Codecov Report

Merging #642 (e1db916) into master (fa8d0f8) will increase coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
+ Coverage   67.17%   67.23%   +0.05%     
==========================================
  Files          53       53              
  Lines        3906     3903       -3     
  Branches     1978     1977       -1     
==========================================
  Hits         2624     2624              
+ Misses       1281     1278       -3     
  Partials        1        1              

Impacted Files	Coverage Δ
src/library.cpp	78.26% <0.00%> (+0.53%)	⬆️
src/libxml_dumper.cpp	0.00% <0.00%> (ø)

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