Minor reformatting in valops.c::value_slice

I noticed that a small lexical block was over indented by 2 characters.
So this patch starts by reducing the indentation.

While looking at this area of the code, I also noticed a couple of lines
that had trailing spaces, so this patch also removes them.

And finally, it fixes one tiny to put the assignment operator at
the start of the next line, rather than at the end of the first line.

gdb/ChangeLog:

        * valops.c (value_slice): Minor reformatting.