Add drop guard to prevent running child process

[jfrimmel]

As noted in this line:

cargo-valgrind/src/main.rs

Line 99 in 2da5213

// TODO: use drop guard, that waits on child in order to prevent printing to stdout of the child

the program needs a drop guard for cases, when the main program dies (e.g. it was killed with Ctrl+C). The current behavior is, that the program exits, but the valgrind child-process remains alive and prints to the standard output of the main process.