riscv: kvm: Fix out-of-bounds array access

In the Linux kernel, the following vulnerability has been resolved: riscv: kvm: Fix out-of-bounds array access In kvm_riscv_vcpu_sbi_init() the entry->ext_idx can contain an out-of-bound index. This is used as a special marker for the base extensions, that cannot be disabled. However, when traversing the extensions, that special marker is not checked prior indexing the array. Add an out-of-bounds check to the function.