Fix crashes in gerber output and excellon parser code

[neinseg]

am_expression.py will crash with gerber files that contain comments like these, generated by a recent KiCAD:

[...]
G04 APERTURE LIST*
G04 Aperture macros list*
%AMRoundRect*
0 Rectangle with rounded corners*   <<<< here ====
0 $1 Rounding radius*
0 $2 $3 $4 $5 $6 $7 $8 $9 X,Y pos of 4 corners*
0 Add a 4 corners polygon primitive as box body*
4,1,4,$2,$3,$4,$5,$6,$7,$8,$9,$2,$3,0*
0 Add four circle primitives for the rounded corners*
1,1,$1+$1,$2,$3*
1,1,$1+$1,$4,$5*
[...]

Also, excellon.py did not work for me. This patch makes it not crash during parsing, but please review my changes--I was unsure of how this code was meant to work in the first place.