When deploying a Hydra instance with current Nix master, most builds
would not run because of errors like this:

  queue monitor: error: --- Error --- hydra-queue-runner
  error: --- UsageError --- nix-daemon
  not a content address because it is not in the form '<prefix>:<rest>': /nix/store/...-somedrv

The last error message is from parseContentAddress, which expects a
colon-separated string, however what we got here is a store path.

Looking at the worker protocol, the following message sent to the Nix
daemon caused the error above:

  0x1E -> wopQuerySubstitutablePathInfos
  0x01 -> Number of paths
  0x16 -> Length of string
  "/nix/store/...-somedrv"
  0x00 -> Length of string
  ""

Looking at writeStorePathCAMap, the store path is indeed the first field
that's transmitted. However, readStorePathCAMap expects it to be the
*second* field *on my machine*, since expression evaluation order is a
classic form of unspecified behaviour[1] in C++.

This has been introduced in #3689,
specifically in commit 66a62b3.

[1]: https://en.wikipedia.org/wiki/Unspecified_behavior#Order_of_evaluation_of_subexpressions

Signed-off-by: aszlig <aszlig@nix.build>

Fix unspecified behaviour in readStorePathCAMap