* malloc/malloc.c (tcache_entry): Add key field. (tcache_put): Set it. (tcache_get): Likewise. (_int_free): Check for double free in tcache. * malloc/tst-tcfree1.c: New. * malloc/tst-tcfree2.c: New. * malloc/Makefile: Run the new tests. * manual/probes.texi: Document memory_tcache_double_free probe. * dlfcn/dlerror.c (check_free): Prevent double frees.