fix out-of-bounds read in rastertolabel delta-row compare

[aizu-m]

Building rastertolabel under ASan and pushing two identical non-blank raster lines to an INTELLITECH_PCL queue:

ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 1 at ... thread T0
    #0 PCLCompress rastertolabel.c:956
0 bytes after 16-byte region allocated by malloc (cupsBytesPerLine)

PCLCompress delta-row compresses each line against the previous one in LastBuffer. Buffer and LastBuffer are both malloc(cupsBytesPerLine). The two compare loops read *line_ptr/*seed first and test line_ptr < line_end second, so a run that reaches the end of the line re-reads one byte past both allocations before the loop exits. It fires whenever consecutive lines share a trailing run, not only on wholly identical lines, so most multi-line pages hit it.

Reordered both conditions to bound-check before the dereference. Valid input compresses to the same bytes.