[C#] Fix script warnings.

Author: mjpt777

csharp/pack.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 DIR="$(dirname "${BASH_SOURCE[0]}")"
-cd $DIR
+cd "${DIR}" || exit
 (cd .. && ./gradlew build)
 (cd sbe-dll && dotnet pack -c release -o ../nuget)

csharp/runtests.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 DIR="$(dirname "${BASH_SOURCE[0]}")"
-cd $DIR
+cd "${DIR}" || exit
 (cd ../; ./gradlew runExampleUsingGeneratedStub -Dsbe.encoding.filename=csharp/sbe-tests/ExampleUsingGeneratedStub.sbe)
 (cd sbe-tests && dotnet test)