test-read-cache: drop namelen variable

test-read-cache: drop namelen variable

Early in the function we set "namelen = strlen(name)" if "name" is
non-NULL. Later, we use "namelen" only if "name" is non-NULL. However,
it's hard to immediately see this, and it seems to confuse gcc 9.2.1
(with "-flto" interestingly, though all of the involved logic is in
inline functions; it also triggers when building with ASan).

Let's simplify the code and remove the variable entirely. There's only
one use of namelen anyway, so we can just call strlen() then. It's true
this is in a loop, so we might execute strlen() more often. But:

  - this is test code that only ever loops twice in our test suite (we
    do loop 1000 times in a t/perf test, but without using this option).

  - a decent compiler ought to be able to hoist that out of the loop
    anyway (though I wouldn't count on gcc 9.2.1 doing so!)

Reported-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>