Double free() in libc or gdb ?

Dear all,

I'm currently having some trouble with the dynamic loader.

I have the libc compilled with "MALLOC_DEBUG" flag to detect double free.
When i run this piece of code (attached file) thought GDB, i have this 
assertion :

Assertion failed: ((run->regs_mask[elm] & (1U << bit)) == 0), function 
arena_run_reg_dalloc, file /usr/src/lib/libc/stdlib/malloc.c, line 2543.

But when i run the same binary without GDB, no assert.

I'm very confused. Can you help me to debug that ?

Best regards

-- 
Alexandre Martins
NETASQ -- We secure IT