draw_struct crashes with a "weird" sequence

[lorenzo-rovigatti]

Hi! Thanks for the lib, it's great.

I was trying to draw a very short sequence and the lib refused to do so, which is fine since the sequence and its secondary structure is non-physical, but I think the code should die gracefully rather than crash. Here is a way of reproducing this issue:

from ipynb.draw import draw_struct

sequence = "GCACGACG"
dot_parens = "().().()"
draw_struct(sequence, dot_parens)

This is the traceback I get:

Error occured while drawing RNA 1 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/lorenzo/software/anaconda3/lib/python3.11/site-packages/draw_rna/ipynb_draw.py", line 27, in draw_struct
    d.draw_rna(seq, secstruct, seq2col(seq), line=line, cmap_name = cmap, vmin=vmin, vmax=vmax,
  File "/home/lorenzo/software/anaconda3/lib/python3.11/site-packages/draw_rna/draw.py", line 62, in draw_rna
    r.setup_tree(secstruct, NODE_R, PRIMARY_SPACE, PAIR_SPACE, external_multiplier, external_offset)
  File "/home/lorenzo/software/anaconda3/lib/python3.11/site-packages/draw_rna/render_rna.py", line 232, in setup_tree
    add_nodes_recursive(bi_pairs,self.root_, jj, bi_pairs[jj])
  File "/home/lorenzo/software/anaconda3/lib/python3.11/site-packages/draw_rna/render_rna.py", line 71, in add_nodes_recursive
    add_nodes_recursive(bi_pairs, newnode, start_index+1, end_index-1)
  File "/home/lorenzo/software/anaconda3/lib/python3.11/site-packages/draw_rna/render_rna.py", line 62, in add_nodes_recursive
    sys.exit(0)
    ^^^
NameError: name 'sys' is not defined