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.

gas/ChangeLog

@@ -1,3 +1,8 @@
+2020-05-27  Simon Cook  <simon.cook@embecosm.com>
+
+        * config/tc-riscv.c (riscv_init_csr_hash): NULL initilize next
+        pointer when creating struct riscv_csr_extra.
+
 2020-05-26  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* testsuite/gas/i386/align-branch-9.d: Updated for PECOFF.

gas/config/tc-riscv.c

@@ -695,6 +695,7 @@ riscv_init_csr_hash (const char *name,
   entry->address = address;
   entry->define_version = define_version;
   entry->abort_version = abort_version;
+  entry->next = NULL;
 
   /* If the CSR hasn't been inserted in the hash table, then insert it.
      Otherwise, attach the extra information to the entry which is already