RISC-V: Fix missing initialization of riscv_csr_extra structs

RISC-V: Fix missing initialization of riscv_csr_extra structs

The next pointer of struct riscv_csr_extra was not always initilized
to NULL or a valid pointer, causing the assembler to attempt to read
through an uninitialized pointer on startup.

gas/ChangeLog:

        * config/tc-riscv.c (riscv_init_csr_hash): NULL initilize next
        pointer when creating struct riscv_csr_extra.