riscv: Fix crash when syscall argument reg index is too high

riscv: Fix crash when syscall argument reg index is too high

By default, doSyscall gets the values of six registers to be used for
system call arguments.  RISC-V, by convention, only has four.  Because
RISC-V's implementation of these indices is as arrays of integers rather
than as base indices plus offsets, trying to get the fifth argument
register's value will cause a crash.  This patch fixes that by returning 0
for any index higher than 3.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>