Certain deltas crash IUP optimisation

[Hoolean]

Hello again - we observed one other issue when trying to experimentally compile one of our larger library fonts with fontc at Dalton Maag.

A particular arrangement of deltas succeeds with fontmake but crashes fontc compilation with an IUP error:

[... ThreadId(1) fontc ERROR] IUP error for glyph: AchievedInvalidState("No best solution identified")
error: process didn't exit successfully: `...\fontc.exe .../Deltas.designspace` (exit code: 1)

Reproducing

As with the earlier issue (#1115), the original font is proprietary and so we've made an independent minimal reproducer to share and use here.

• ZIP containing designspace and UFO
• Original script used to construct them:

from fontTools.designspaceLib import (
    AxisDescriptor,
    DesignSpaceDocument,
    InstanceDescriptor,
    SourceDescriptor,
)
from ufoLib2 import Font
from ufoLib2.objects import Contour, Point

# Deltas that trigger the IUP crash
# (more minimal deltas are probably possible, but at this point I struggled to
#  use fewer while still triggering the crash)
DELTAS = [
    Point(x=0, y=0, type="line"),
    Point(x=1, y=0, type="line"),
    Point(x=1, y=0, type="line"),
    Point(x=0, y=1, type="line"),
    Point(x=0, y=0, type="line"),
    Point(x=1, y=0, type="line"),
    Point(x=1, y=3, type="line"),
    Point(x=2, y=0, type="line"),
    Point(x=2, y=3, type="line"),
    Point(x=0, y=0, type="line"),
    Point(x=0, y=0, type="line"),
    Point(x=0, y=3, type="line"),
]

# Create designspace with weight axis
doc = DesignSpaceDocument()
doc.addAxis(
    AxisDescriptor(tag="wght", name="Weight", minimum=400, maximum=700, default=400)
)

# Create a glyph with all points at origin in one source, and all points at
# particular offsets in another.
for style, weight, points in [
    ("Regular", 400, DELTAS),
    ("Bold", 700, [Point(x=0, y=0, type="line") for _ in range(len(DELTAS))]),
]:
    ufo = Font()

    inner = ufo.newGlyph("glyph")
    inner.contours.append(Contour(points))

    ufo.save(f"{style}.ufo", overwrite=True)

    doc.addSource(SourceDescriptor(path=f"{style}.ufo", location={"Weight": weight}))
    doc.addInstance(InstanceDescriptor(styleName=style, location={"Weight": weight}))

doc.write("Deltas.designspace")

[Hoolean]

(this is a stab in the dark as I'm not able to investigate further at this time, but could this conditional before the error have an off-by-one? it looks like <= might be needed instead of < to only catch the cases where the index would be negative)