fix CppCheck warning in string-utilities.h

[maloel]

common\utilities\string\string-utilities.h:118:26: error: Uninitialized variable: last_char_idx [uninitvar]
    return str.size() == last_char_idx; // all the chars in the string are converted to the value (thus there are no other chars other than numbers)
                         ^

last_char_idx was uninitialized.
The file has a ton of tabs, too -- clang'ed it, sorry :)

[Nir-Az]

Looks good