test: add libFuzzer harness for rrd_restore

[somethingwithproof]

Extends the existing rrd_open fuzz harness (#1368) to rrd_restore's XML
parsing path -- the original ask named both as fuzz targets, only
rrd_open was covered. Opt-in via --enable-fuzzing, not part of the
default build or make check.

Found a real, independently-confirmed heap-buffer-overflow read within
a 30s smoke run using only the seed corpus -- fixed separately in #1371,
which also has the before/after verification. The crash-triggering
fixture is kept in the corpus here as a regression seed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.65%. Comparing base (be32943) to head (8c8b968).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1372      +/-   ##
==========================================
+ Coverage   51.21%   51.65%   +0.43%     
==========================================
  Files          45       45              
  Lines       18372    18420      +48     
==========================================
+ Hits         9410     9514     +104     
+ Misses       8962     8906      -56     

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