When accessing a function pointer to a static closed delegate like
done in added test:

GetFunctionPointerForDelegate_MarshalledClosedStaticDelegate

it will trigger a read outside of the allocated mspecs buffer since
invoke_sig and method signature arguments wont match.

There was already logic to adjust this in emit_managed_wrapper_ilgen,
but done after call to emit_managed_wrapper_validate_signature
that will touch memory and depending on its content, trigger a crash.

Fix make sure we do signature adjustments first and then validate
the signature. Fix also adjust a couple of mspecs allocations to use
g_new0 as all others to make sure we get NULL pointers in the mspecs
array.

Since this scenario was not covered on CI, commit also adds a new test
in GetFunctionPointerForDelegateTests.cs covering this scenario.