The additional segfault mentioned in #244 was due to the fact that
the merged upsamplers use a different private structure than the
non-merged upsamplers. jpeg_skip_scanlines() was assuming the latter, so
when merged upsampling was enabled, jpeg_skip_scanlines() clobbered one
of the IDCT method pointers in the merged upsampler's private structure.
For reasons unknown, the test image in #441 did not encounter this
segfault (too small?), but it encountered an issue similar to the one
fixed in 5bc43c7821, whereby it was
necessary to set up a dummy postprocessing function in
read_and_discard_scanlines() when merged upsampling was enabled.
Failing to do so caused either a segfault in merged_2v_upsample() (due
to a NULL pointer being passed to jcopy_sample_rows()) or an error
("Corrupt JPEG data: premature end of data segment"), depending on the
number of scanlines skipped and whether the first scanline skipped was
an odd- or even-numbered row.
Fixes#441Fixes#244 (for real this time)
