Prevent use-after-free when adding subcommand aliases

The add_subcommand_alias routine contained a use-after-free bug
which was discovered by valgrind. I actually used the AddressSanitizer
to track down this bug - it is much more useful.

The actual bug is that old_sc saved a reference to the old array
that was realloc-ed and then the function pointer from out the now
free'ed array was copied over.

Fix this by storing the function pointer into a temporary, then
realloc-ing the array and then copy the temporary value into the
new array entry.

Github-Issue: #122
Reported-By: Mikael Fangel (https://github.com/MikaelFangel)

src/cmd/gcli.c

@@ -269,6 +269,7 @@ add_subcommand_alias(char const *alias_name, char const *alias_for)
 	char *docstring;
 	struct subcommand const *old_sc;
 	struct subcommand *new_sc;
+	int (*old_fn)(int, char **);
 
 	old_sc = find_subcommand(alias_for, NULL);
 	if (old_sc == NULL) {
@@ -277,14 +278,15 @@ add_subcommand_alias(char const *alias_name, char const *alias_for)
 		exit(EXIT_FAILURE);
 	}
 
+	old_fn = old_sc->fn;
 	docstring = sn_asprintf("Alias for %s", alias_for);
 	subcommands = realloc(subcommands, (subcommands_size + 1) * sizeof(*subcommands));
 
 	/* Copy in data */
 	new_sc = &subcommands[subcommands_size++];
 
 	new_sc->cmd_name = alias_name;
-	new_sc->fn = old_sc->fn;
+	new_sc->fn = old_fn;
 	new_sc->docstring = docstring;
 }