gdb/riscv: Fix oob memory access when printing info registers

If the length of a register name was greater than 15,
print_spaces was called with a negative number, which
prints random data from the heap instead of the requested
number of spaces.

This could happen if a target-description file was used
to specify additional long-named registers.

Fix is simple - don't ask for fewer than 1 space (since
we still want column separation).

Approved-by: Kevin Buettner <kevinb@redhat.com>