`assert current_scan_base > 0`

[AlexDenisov]

filecheck hits an assertion in the following case (reduced as much I could):

echo "3" > input.txt
echo "[2]" >> input.txt
echo "// CHECK-NEXT: 2" > assert.c
cat input.txt | filecheck assert.c

Stacktrace:

/Library/Frameworks/Python.framework/Versions/3.7/bin/filecheck
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/bin/filecheck", line 10, in <module>
    sys.exit(main())
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/filecheck/FileCheck.py", line 589, in main
    assert current_scan_base > 0
AssertionError