Memory leak on Crabstone::Instruction

The Instruction class is not properly released due to a typo in the release class method. While no warnings or errors will be raised when the finalizer is called automatically, attempting to manually free the structure will tell you that the read_ptr method doesn't exist and suggest read_pointer.

Please refer to this commit for the fix: https://github.com/kdiv/crabstone/commit/a7bb211a59748e46dde7f4401d83c33ebe19a563

[david942j]

@kdiv This bug is fixed in my fork, where is planned to support Capstone 4.0