Invalid data written to stream when runway is used

[arnetheduck]

In the category of strange errors, status-im/nim-snappy#11 shows how random data gets written to a stream for some nim versions when optimizations are turned on.

This smells of reliance on some undefined behavior that make some compilers / compiler options emit unexpected code.

The bug goes away if ensureRunway is not called.

Cannot repro locally on fedora / gcc 11.2.1.