Fix a very unlikely GC bug when using a callback block

[larskanis]

This could happen, when the Proc object created by rb_block_proc() is GC'ed before the call is executed. Practically this is very unlikely unless called with GC.stress=true .

It can easily be fixed, by returning the Proc object and keeping it in a local variable while the call is executed.

When running the test suite with GC.stress=true then it failed like so: Failures:

  1) Callback struct by value parameter
     Failure/Error:
       LibTest.testCallbackTrV(s) do |struct|
         s2[:s8] = struct[:s8]
         s2[:f32] = struct[:f32]
         s2[:s32] = struct[:s32]
       end

     NoMethodError:
       undefined method `call' for an instance of FFI::MemoryPointer
      ./spec/ffi/callback_spec.rb:312:in `testCallbackTrV'
      ./spec/ffi/callback_spec.rb:312:in `block (2 levels) in <module:CallbackSpecs>'

  2) Callback with  struct by reference argument
     Failure/Error: LibTest.testCallbackYrV(magic) { |i| v = i }

     NoMethodError:
       undefined method `call' for an instance of FFI::Pointer
      ./spec/ffi/callback_spec.rb:802:in `testCallbackYrV'
      ./spec/ffi/callback_spec.rb:802:in `block (2 levels) in <module:CallbackWithSpecs>'

  3) Callback with  struct by reference argument with nil value
     Failure/Error: LibTest.testCallbackYrV(nil) { |i| v = i }

     NoMethodError:
       undefined method `call' for an instance of CallbackWithSpecs::LibTest::S8F32S32
      ./spec/ffi/callback_spec.rb:809:in `testCallbackYrV'
      ./spec/ffi/callback_spec.rb:809:in `block (2 levels) in <module:CallbackWithSpecs>'

Finished in 100 minutes 21 seconds (files took 0.56651 seconds to load) 5014 examples, 3 failures