Fix node.compile fwrite size check.

[h2zero]

• [X] This PR is for the dev branch rather than for the release branch.
• [X] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [X] I have thoroughly tested my contribution.
• [ ] The code changes are reflected in the documentation at docs/*.

The return value from fwrite was being checked against the size of the data rather than the number of bytes written. This caused node.compile() to falsely return failure.

[jmattsson]

Nice catch, thanks!