tls: Prevent hang in readStart

This is not a great fix, and it's a bug that's very tricky to reproduce.

Occasionally, while downloading a file, especially on Linux for some
reason, the pause/resume timing will be just right such that the
CryptoStream is in a 'reading' state, but actually has no data, so it
ought to pull more in.  Because there's no reads happening, it just sits
there, and the process will exit

This is, fundamentally, a factor of how the HTTP implementation sits
atop CryptoStreams and TCP Socket objects, which is utterly horrible,
and needs to be rewritten.  However, in the meantime, npm downloads are
prematurely exiting, causing hard-to-debug "cb() never called!" errors.