crypto: lib/mpi - avoid null pointer deref in mpi_cmp_ui()

In the Linux kernel, the following vulnerability has been resolved: crypto: lib/mpi - avoid null pointer deref in mpi_cmp_ui() During NVMeTCP Authentication a controller can trigger a kernel oops by specifying the 8192 bit Diffie Hellman group and passing a correctly sized, but zeroed Diffie Hellamn value. mpi_cmp_ui() was detecting this if the second parameter was 0, but 1 is passed from dh_is_pubkey_valid(). This causes the null pointer u->d to be dereferenced towards the end of mpi_cmp_ui()