riscv, bpf: Fix potential NULL dereference

In the Linux kernel, the following vulnerability has been resolved:

riscv, bpf: Fix potential NULL dereference

The bpf_jit_binary_free() function requires a non-NULL argument. When

the RISC-V BPF JIT fails to converge in NR_JIT_ITERATIONS steps,

jit_data->header will be NULL, which triggers a NULL

dereference. Avoid this by checking the argument, prior calling the

function.