Shapely: segmentation fault from LineString.interpolate()

@snorfalorpagus, here you go:

Expected behavior and actual behavior.

Should throw a traceback or warning when attempting to interpolate a point along a LINESTRING EMPTY object, but instead crashes the kernel.

Steps to reproduce the problem

• gist in notebook that results in a dead kernel (and see .png end of issue report)
• this code chunk (below) results in a segmentation fault:

# python shapely_interpolate_segmentation_fault.py
import faulthandler
faulthandler.enable()
from shapely.geometry import Point, LineString
test_line = LineString(((0,0), (1,1), (2,1)))
print(test_line)
empty_line = test_line.parallel_offset(10., side='right')
print(empty_line)
empty_line.interpolate(.5, normalized=True)

$ python shapely_interpolate_segmentation_fault.py
LINESTRING (0 0, 1 1, 2 1)
LINESTRING EMPTY
Fatal Python error: Segmentation fault

Current thread 0x000000010ce7f5c0 (most recent call first):
  File "/Users/user/miniconda3/envs/py3_at1866/lib/python3.6/site-packages/shapely/linref.py", line 22 in __call__
  File "/Users/user/miniconda3/envs/py3_at1866/lib/python3.6/site-packages/shapely/geometry/base.py", line 777 in interpolate
  File "/Users/user/miniconda3/envs/py3_at1866/lib/python3.6/site-packages/shapely/impl.py", line 37 in wrapper
  File "shapely_interpolate_segmentation_fault.py", line 9 in <module>
Segmentation fault: 11

Operating system

• Mac OS X 10.14

Shapely version and provenance

• 1.6b4 installed from conda