This only moves the code blocks, but does not change any code at all.

Make sure calls to GEN_ALLOC_ready always follow GEN_ALLOC_readyplus.

Introduce an internal static helper function that does the common parts, which
both functions can use.

While at it drop the register specification.

The names of the function parameters as well as the name of the local variable
were taken from the macro arguments. Noone passed anything different than the
default names there, and doing so doesn't make any sense anyway.

Save the new length in a local variable, and only change the value in the struct
if allocation succeeds instead of changing it back on failure.

This was doing 2 assignments, a function call and a return on the same line.
This had the additional slight bug that if the allocation failed it returned 0,
but the allocation length in the struct passed in was already changed to the new
value. This would not do any harm on subsequent calls as the pointer was still
NULL and only that is checked.

alloc() is used in the generated code, so directly include it in this header
instead of all the callers.