[1820893135-pixel]

Fixes #822.

Root cause

rndr_tablerow() calls bufput(ob, text->data, text->size) for an empty cell,
which passes data == NULL and len == 0. bufput() then calls
memcpy(dst, NULL, 0); the source of memcpy() is declared nonnull, so this
is undefined behavior even though no bytes move:

buffer.c:169:32: runtime error: null pointer passed as argument 2, which is declared to never be null

With -fno-sanitize-recover=all, the one-byte document containing a single
pipe character aborts.

Fix

Return early when len == 0, before the grow and the copy.

Verification

Build (no Ruby needed):

D=ext/redcarpet
clang -g -O1 -D_GNU_SOURCE -std=gnu11 -fsanitize=address,undefined \
  -fno-sanitize-recover=all -I$D driver.c \
  $D/markdown.c $D/html.c $D/html_smartypants.c $D/autolink.c \
  $D/buffer.c $D/stack.c $D/houdini_href_e.c $D/houdini_html_e.c -o repro

with the driver.c from the report.

input	bytes	before	after
a single pipe character	1	exit 1, UBSan buffer.c:169	exit 0

No behaviour change: rendered 71 documents (every
test/MarkdownTest_1.0*/Tests/*.text plus 30 autolink/table/blockquote cases)
with the pristine and the patched build (plain -O2); HTML output, stderr and
exit status are byte-identical.