Fix out-of-bounds access in sanitizeSlashes

[mjcarroll]

Fixes #301

Before this fix:

• Expected behavior: Tests build and run when -D_GLIBCXX_ASSERTIONS is set
• Actual behavior: Test aborts due to out-of-bounds access

Steps to reproduce

git checkout ign-common4
CXXFLAGS=-D_GLIBCXX_ASSERTIONS cmake -B build -S . -DCMAKE_BUILD_TYPE=Debug
make -C build UNIT_Filesystem_TEST
./build/bin/UNIT_Filesystem_TEST
gdb -ex run ./build/bin/UNIT_Filesystem_TEST

Signed-off-by: Michael Carroll michael@openrobotics.org

[codecov]

Codecov Report

Merging #303 (b9e4cc0) into ign-common3 (af625e2) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           ign-common3     #303   +/-   ##
============================================
  Coverage        76.58%   76.58%           
============================================
  Files               73       73           
  Lines            10403    10403           
============================================
  Hits              7967     7967           
  Misses            2436     2436           

Impacted Files	Coverage Δ
src/Filesystem.cc	76.11% <100.00%> (ø)

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