pbx.c: prevent potential crash from recursive replace()

Recurisve usage of replace() resulted in corruption of the
temporary string storage and potential crash.  By changing
the string to be allocated separtely per instance, this is
eliminated.

ASTERISK-23650 #comment Reported by: Roel van Meer
ASTEIRSK-23650 #close

Review: https://reviewboard.asterisk.org/r/3539/
........

Merged revisions 414214 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@414215 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_strings.c

@@ -798,7 +798,7 @@ static int replace(struct ast_channel *chan, const char *cmd, char *data, struct
 		AST_APP_ARG(replace);
 	);
 	char *strptr, *varsubst;
-	struct ast_str *str = ast_str_thread_get(&result_buf, 16);
+	RAII_VAR(struct ast_str *, str, ast_str_create(16), ast_free);
 	char find[256]; /* Only 256 characters possible */
 	char replace[2] = "";
 	size_t unused;