QuickJS: free joined string with chain pool in qjs_string_create_chb()

QuickJS: free joined string with chain pool in qjs_string_create_chb()

Previously, the joined string was freed with the JSContext passed as
the pool argument of the chain free function.  This only worked for
chains initialized with NJS_CHB_CTX_INIT(), where the pool is the
context itself.

Now, the chain pool is used, so the function works with chains using
any allocator.  No functional changes for existing callers.