Patch native compilation warnings.

[GICodeWarrior]

Fixes these warnings.

> node-gyp rebuild

  CXX(target) Release/obj.target/validation/src/validation.o
../src/validation.cc:71:21: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  for (int i = 0; i < len; i++) {
                  ~ ^ ~~~
../src/validation.cc:75:24: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    if (extrabytes + i >= len)
        ~~~~~~~~~~~~~~ ^  ~~~
../src/validation.cc:144:1: warning: no newline at end of file [-pedantic,-Wnewline-eof]

^

[theturtle32]

Sweet thank you!