bignum.cpp: Avoid long to double conversion under conditional test

[spchamp]

This patch adds an additional cast for a conditional test form, in the definition of Bignum::from_double in machine/class/bignum.cpp

The cast should serve to ensure that the local variable value will be evaluated as being of a long type, within the left hand expression in the conditional test form. clang++ 11 has detected a potential type incompatibility, in this expression.

The value is already being cast to long, in the right hand expression in the test form, i.e 0 != (long)value.

Previous to the patch, compiling with clang++ 11 on FreeBSD 11.1

2: CXX machine/class/bignum.cpp
4: CXX machine/class/block_as_method.cpp
machine/class/bignum.cpp:1112:32: error: implicit conversion from 'long' to 'double' changes value from 4611686018427387903 to 4611686018427387904 [-Werror,-Wimplicit-const-int-float-convers>
    while(!(value <= (LONG_MAX >> 1)) || 0 != (long)value) {
                  ~~  ~~~~~~~~~^~~~
3: CXX machine/class/block_environment.cpp
1 error generated.
Error: /usr/local/bin/clang++11 -I/usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/machine -I/usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/mach>
/usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/build/scripts/daedalus.rb:69:in `command': Error compiling (RuntimeError)
        from /usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/build/scripts/daedalus.rb:235:in `cxx_compile'
        from /usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/build/scripts/daedalus.rb:222:in `compile'
        from /usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/build/scripts/daedalus.rb:438:in `build'
        from /usr/home/gimbal/wk/dist_wk/ports_devo/lang/rubinius/work/rubinius-5.0/build/scripts/daedalus.rb:958:in `block (2 levels) in perform_tasks'

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1. Is this pull-request complete?

• Yes, this pull-request is ready to be reviewed and merged.
• No, this pull-request is a work-in-progress.

2. Does this pull request fix an issue with an existing feature or introduce a new feature?

• It fixes an issue with an existing features.
• It introduces a new feature.

3. Does this pull-request include tests?

• Yes, it includes tests.
• No, it does not include tests because the tests already exist.
• No, it does not include tests because tests are too difficult to write.
• No, it does not include tests because ...