The TIFFPrintDirectory function relies on the following assumptions,
supposed to be guaranteed by the specification:

(a) A Transfer Function field is only present if the TIFF file has
    photometric type < 3.

(b) If SamplesPerPixel > Color Channels, then the ExtraSamples field
    has count SamplesPerPixel - (Color Channels) and contains
    information about supplementary channels.

While respect of (a) and (b) are essential for the well functioning of
TIFFPrintDirectory, no checks are realized neither by the callee nor
by TIFFPrintDirectory itself. Hence, following scenarios might happen
and trigger the NULL pointer dereference:

(1) TIFF File of photometric type 4 or more has illegal Transfer
    Function field.

(2) TIFF File has photometric type 3 or less and defines a
    SamplesPerPixel field such that SamplesPerPixel > Color Channels
    without defining all extra samples in the ExtraSamples fields.

In this patch, we address both issues with respect of the following
principles:

(A) In the case of (1), the defined transfer table should be printed
    safely even if it isn't 'legal'. This allows us to avoid expensive
    checks in TIFFPrintDirectory. Also, it is quite possible that
    an alternative photometric type would be developed (not part of the
    standard) and would allow definition of Transfer Table. We want
    libtiff to be able to handle this scenario out of the box.

(B) In the case of (2), the transfer table should be printed at its
    right size, that is if TIFF file has photometric type Palette
    then the transfer table should have one row and not three, even
    if two extra samples are declared.

In order to fulfill (A) we simply add a new 'i < 3' end condition to
the broken TIFFPrintDirectory loop. This makes sure that in any case
where (b) would be respected but not (a), everything stays fine.

(B) is fulfilled by the loop condition
'i < td->td_samplesperpixel - td->td_extrasamples'. This is enough as
long as (b) is respected.

Naturally, we also make sure (b) is respected. This is done in the
TIFFReadDirectory function by making sure any non-color channel is
counted in ExtraSamples.

This commit addresses CVE-2018-7456.