Bug 5317: FATAL attempt to read data from memory (#1579)

Bug 5317: FATAL attempt to read data from memory (#1579)

    FATAL: Squid has attempted to read data ... that is not present.

Recent commit 122a6e3 attempted to deliver in-memory response body bytes
to a Store-reading client that requested (at least) response headers.
That optimization relied on the old canReadFromMemory() logic, but that
logic results in false positives when the checked read offset falls into
a gap between stored headers and the first body byte of a Content-Range.
In that case, a false positive leads to a readFromMemory() call and a
FATAL mem_hdr::copy() error.

This workaround disables the above optimization without fixing
canReadFromMemory(). We believe that a readFromMemory() call that comes
right after response headers are delivered to the Store-reading client
will not suffer from the same problem because the client will supply the
read offset of the first body byte, eliminating the false positive.