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

(cherry picked from commit 750333ae03)

main/asterisk.c

@@ -1118,6 +1118,10 @@ void ast_unreplace_sigchld(void)
 	unsigned int level;
 
 	ast_mutex_lock(&safe_system_lock);
+
+	/* Wrapping around here is an error */
+	ast_assert(safe_system_level > 0);
+
 	level = --safe_system_level;
 
 	/* only restore the handler if we are the last one */