argument: Slightly simplify option parsing logic

Do so to fix cppcoreguidelines-pro-bounds-pointer-arithmetic and
readability-implicit-bool-conversion:

```
../argument.cpp:43:13: error: do not use pointer arithmetic [cppcoreguidelines-pro-bounds-pointer-arithmetic,-warnings-as-errors]
            ++i;
            ^
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../argument.cpp:46:13: error: implicit conversion 'int' -> bool [readability-implicit-bool-conversion,-warnings-as-errors]
        if (i->val)
            ^
                   != 0
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../argument.cpp:48:35: error: implicit conversion 'int' -> bool [readability-implicit-bool-conversion,-warnings-as-errors]
            arguments[i->name] = (i->has_arg ? optarg : true_string);
                                  ^
                                             != 0
```

Change-Id: If3d950d17e9f8117246cffdb9305262a18cf8aa1
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
2 files changed