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commit 12d41a02 upstream.

When setting the secret with the software Diffie-Hellman implementation,
if allocating 'g' failed (e.g. if it was longer than
MAX_EXTERN_MPI_BITS), then 'p' was freed twice: once immediately, and
once later when the crypto_kpp tfm was destroyed.

Fix it by using dh_free_ctx() (renamed to dh_clear_ctx()) in the error
paths, as that correctly sets the pointers to NULL.

KASAN report:

    MPI: mpi too large (32760 bits)
    ==================================================================
    BUG: KASAN: use-after-free in mpi_free+0x131/0x170
    Read of size 4 at addr ffff88006c7cdf90 by task reproduce_doubl/367

    CPU: 1 PID: 367 Comm: reproduce_doubl Not tainted 4.14.0-rc7-00040-g05298abde6fe #7
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
    Call Trace:
     dump_stack+0xb3/0x10b
     ? mpi_free+0x131/0x170
     print_address_description+0x79/0x2a0
     ? mpi_free+0x1...