Remove unnecessary assertion which caused a crash with nested here docs.

Uncovered by wild tests.

osh/cmd_parse.py

@@ -40,7 +40,6 @@ def _ReadHereLines(line_reader, h, delimiter):
  last_line = None
  while True:
    line_id, line, unused_offset = line_reader.GetLine()
    assert unused_offset == 0

    if not line:  # EOF
      # An unterminated here doc is just a warning in bash.  We make it

spec/here-doc.test.sh

@@ -324,6 +324,19 @@ here doc in IF CONDITION
THEN executed
## END

#### Nested here docs which are indented
cat <<- EOF
	outside
	$(cat <<- INSIDE
		inside
INSIDE
)
EOF
## STDOUT:
outside
inside
## END

#### Multiple here docs in pipeline
# SKIPPED: hangs with osh on Debian
# The second instance reads its stdin from the pipe, and fd 5 from a here doc.
@@ -350,3 +363,4 @@ EOF5
0: 3: fd3
5: fd5
## END

test/spec.sh

@@ -400,7 +400,7 @@ here-doc() {
  # - On Debian, the whole process hangs.
  # Is this due to Python 3.2 vs 3.4?  Either way osh doesn't implement the
  # functionality, so it's probably best to just implement it.
  sh-spec spec/here-doc.test.sh --range 0-30 \
  sh-spec spec/here-doc.test.sh --range 0-31 \
    ${REF_SHELLS[@]} $OSH_LIST "$@"
}