Fix check for negative pid (#888)

The pid variable is a pointer, and pointers cannot be negative. Fix
the check to test the value of the pid rather than the address of the
pointer.

src/bin/pg_autoctl/pidfile.c

@@ -193,7 +193,7 @@ read_pidfile(const char *pidfile, pid_t *pid)
 
 	free(fileContents);
 
-	if (pid <= 0)
+	if (*pid <= 0)
 	{
 		log_debug("Read negative pid %d in file \"%s\", removing it",
 				  *pid, pidfile);