[llvm-c-test] Fix warnings

This patch fixes:

  llvm/tools/llvm-c-test/metadata.c:49:16: error: unused variable
  'tmp' [-Werror,-Wunused-variable]

  llvm-project/llvm/tools/llvm-c-test/metadata.c:70:16: error: unused
  variable 'md' [-Werror,-Wunused-variable]

  llvm/tools/llvm-c-test/metadata.c:74:16: error: unused variable
  'md2' [-Werror,-Wunused-variable]