res_agi.c: Ensure SIGCHLD handler functions are properly balanced.

Calls to `ast_replace_sigchld()` and `ast_unreplace_sigchld()` must be
balanced to ensure that we can capture the exit status of child
processes when we need to. This extends to functions that call
`ast_replace_sigchld()` and `ast_unreplace_sigchld()` such as
`ast_safe_fork()` and `ast_safe_fork_cleanup()`.

The primary change here is ensuring that we do not call
`ast_safe_fork_cleanup()` in `res_agi.c` if we have not previously
called `ast_safe_fork()`.

Additionally we reinforce some of the documentation and add an
assertion to, ideally, catch this sooner were this to happen again.

Fixes #922

include/asterisk/app.h

@@ -1491,6 +1491,10 @@ int ast_safe_fork(int stop_reaper);
 
 /*!
  * \brief Common routine to cleanup after fork'ed process is complete (if reaping was stopped)
+ *
+ * \note This must <b>not</b> be called unless ast_safe_fork(1) has been called
+ * previously.
+ *
  * \since 1.6.1
  */
 void ast_safe_fork_cleanup(void);