Fix legend D without offset and G without argument - #9280

Two more small bugs in the legend records, found while auditing pslegend.c (follow-up to #9277). Bugs found by Claude Opus 5.5; fix written by Claude Sonnet 5.5.

This was first pushed directly to master by mistake and reverted in f0d9d18; this PR proposes it again.

1. D 1p + (offset left out): the legend height estimate read it as offset=1p, pen=+, but drawing reads it as pen=1p with flag +. So the automatic height of the box was different from the one for D 0 1p +, which draws exactly the same. The same happens with the flags - and =.
2. G without argument: it read past the end of the record, printing garbage warnings and exiting with code 0. It now reports Not enough arguments given to G operator.

Scripts to see both:

# 1. Both legends draw the same, so both heights should be the same
printf 'S - c 0.3c red 0.5p - A\nD 1p +\nS - c 0.3c blue 0.5p - B\n' | gmt pslegend -Dx0/0+w5c -Vi 2>&1 >/dev/null | grep estimated
printf 'S - c 0.3c red 0.5p - A\nD 0 1p +\nS - c 0.3c blue 0.5p - B\n' | gmt pslegend -Dx0/0+w5c -Vi 2>&1 >/dev/null | grep estimated

# 2. Expected: a clear error and a non-zero exit code
printf 'S - c 0.3c red 0.5p - A\nG\nS - c 0.3c blue 0.5p - B\n' | gmt pslegend -Dx0/0+w5c > /dev/null; echo "exit code: $?"

In 1, master gives 1.446 cm for D 1p + and 1.330 cm for D 0 1p +; with this PR both are 1.330 cm. In 2, master prints warnings with garbage characters and returns 0; now it prints the error and returns an error code.

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