Fix out-of-bound access in URI

[mjcarroll]

Uncovered in the process of testing #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_URI_TEST
./build/bin/UNIT_URI_TEST
gdb -ex run ./build/bin/UNIT_URI_TEST

[codecov]

Codecov Report

Merging #304 (78cc81b) into ign-common4 (d5261a8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           ign-common4     #304   +/-   ##
============================================
  Coverage        77.02%   77.02%           
============================================
  Files               76       76           
  Lines            10727    10729    +2     
============================================
+ Hits              8262     8264    +2     
  Misses            2465     2465           

Impacted Files	Coverage Δ
src/URI.cc	97.42% <100.00%> (+<0.01%)	⬆️

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