db: avoid -Werror=format-truncation with GCC >= 7

[diabonas]

Since commit 548c65c GCC >= 7 produces several warnings of the form

src/lib/db.c: In function ‘db_new.constprop’:
src/lib/db.c:1504:37: error: ‘%s’ directive output may be truncated writing 5 bytes into a region of size between 1 and 4096 [-Werror=format-truncation=]
 1504 |     snprintf(lockpath, PATH_MAX, "%s%s", path, ".lock");
      |                                     ^~         ~~~~~~~

[codecov]

Codecov Report

Merging #415 into master will decrease coverage by 0.08%.
The diff coverage is 47.05%.

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   72.27%   72.19%   -0.09%     
==========================================
  Files          30       30              
  Lines        6446     6459      +13     
==========================================
+ Hits         4659     4663       +4     
- Misses       1787     1796       +9

Impacted Files	Coverage Δ
src/lib/db.c	64.42% <47.05%> (-0.55%)	⬇️

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