[v2] Don't throw an error in 'info registers' for unavailable MIPS registers.

'info registers' for MIPS throws an error and when it first encounters
an unavailable register.  This does not match other architectures
which annotate unavailable registers and continue to print out the
values of subsequent registers.  Replace the error by displaying an
aligned "<unavailable>".  This string is truncated to "<unavl>" when
displaying a 32-bit register.

gdb/ChangeLog:

	* mips-tdep.c (print_gp_register_row): Don't error for unavailable
	registers.
---
 gdb/ChangeLog   |  5 +++++
 gdb/mips-tdep.c | 10 ++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

On Fri, 16 Jun 2017, John Baldwin wrote:

> index 3ff2dab491..adc843a5be 100644
> --- a/gdb/mips-tdep.c
> +++ b/gdb/mips-tdep.c
> @@ -6542,8 +6542,14 @@ print_gp_register_row (struct ui_file *file, struct frame_info *frame,
>        value = get_frame_register_value (frame, regnum);
>        if (value_optimized_out (value)
>  	|| !value_entirely_available (value))
> -	error (_("can't read register %d (%s)"),
> -	       regnum, gdbarch_register_name (gdbarch, regnum));
> +	{
> +	  fprintf_filtered (file, "%*s ",
> +			    (int) mips_abi_regsize (gdbarch) * 2,
> +			    mips_abi_regsize (gdbarch) == 4 ? "<unavl>"
> +			    : "<unavailable>");

 Formatting nit here -- you need to wrap an expression in parentheses when 
splitting between lines and indent accordingly, i.e. either:

+			    (mips_abi_regsize (gdbarch) == 4 ? "<unavl>"
+			     : "<unavailable>"));

or:

+			    (mips_abi_regsize (gdbarch) == 4
+			     ? "<unavl>" : "<unavailable>"));

> +	  col++;
> +	  continue;
> +	}

 Otherwise LGTM, so please consider your patch with such a change applied 
preapproved.

 I think that duplicate `col++' could be avoided, as could be the repeated 
calls to `mips_abi_regsize' and `register_size'.  That can be a separate 
clean-up though.

 Thanks for your work on this problem.

  Maciej