Fixed warning

[MonkeybreadSoftware]

Fixed a warning about possible misuse of comma in the while() with casting it to void.

[MonkeybreadSoftware]

the warning if you like to use it:

In file included from .../jsoncons/include/jsoncons/json_visitor.hpp:24:
.../jsoncons/include/jsoncons/utility/bigint.hpp:1318:18: warning: possible misuse of comma operator here [-Wcomma]
 1318 |                 while ( b >>= 2, b > 0 )
      |                                ^
.../jsoncons/include/jsoncons/utility/bigint.hpp:1318:11: note: cast expression to void to silence warning
 1318 |                 while ( b >>= 2, b > 0 )
      |                         ^~~~~~~
      |                         static_cast<void>( )
1 warning generated.