Check the buffer size *before* calling the continuation in withEncodedCString

This fixes a very subtle bug in withEncodedCString where a reference would be kept to the whole continuation until the continuation had finished executing. This was because the call to tryFillBufferAndCall could fail, if the buffer was already full and so the go helper would be recursively called on failure which necessitated keeping a reference to act.

The failure could only happen during the initial checking phase of the function but not during the call to the continuation. Therefore the fix is to first perform the size check, potentially recursively and then finally calling tail calling the continuation.

In the real world, this broke writing lazy bytestrings because a reference to the head of the bytestring would be retained in the continuation until the whole string had been written to a file.

Fixes #20107 (closed)