Fix #146, Add Null Terminator for FileName in FM_GetDirListFileCmd

[arielswalker]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
Fixes #146

Testing performed
Reran the internal static code analysis to ensure warning was resolved. Used the following commands to build:

git clone https://github.com/nasa/cFS.git
cd cFS
git submodule init
git submodule update
cp -f ./cfe/cmake/Makefile.sample Makefile
rm -rf sample_defs && cp -r ./cfe/cmake/sample_defs sample_defs

Update sample_defs/targets.cmake

LIST(APPEND MISSION_GLOBAL_APPLIST fm)

Run make prep and install

make SIMULATION=native OMIT_DEPRECATED=false BUILDTYPE=debug ENABLE_UNIT_TESTS=false CMAKE_EXPORT_COMPILE_COMMANDS=ON MISSIONCONFIG=sample prep
make install

Expected behavior changes
Resolves static code analysis warning of non-null-terminated-string

System(s) tested on
Internal static code analysis tool, RedHat, cFS dev

Contributor Info - All information REQUIRED for consideration of pull request
Ariel Walker, MCSG TECH.