[PATCH] do not read beyond end of malloc'd buffer

I was surprised to see "git diff --word-diff" output a ton of
garbage, and tracked it down to a bug that's triggered when the
diff.suppress-blank-empty config option to true and when at least
one of the context lines is empty.

Here's a quick demo you can run in an empty directory:

    printf 'a\n\n[-b-]{+c+}\n' > exp
    git init && git config diff.suppress-blank-empty true
    printf 'a\n\nb\n' > f && git add . && git commit -m. .
    printf 'a\n\nc\n' > f
    git diff --word-diff | tail -3 > out
    diff out exp

Before the patch, the git diff ... command would read from beyond
the end of a heap buffer, and "out" would contain far more than the
expected 5 bytes.

Here's the patch:

-- >8 --
Subject: [PATCH] do not read beyond end of malloc'd buffer

With diff.suppress-blank-empty=true, "git diff --word-diff" would
output data that had been read from uninitialized heap memory.
The problem was that fn_out_consume did not account for the
possibility of a line with length 1, i.e., the empty context line
that diff.suppress-blank-empty=true converts from " \n" to "\n".
Since it assumed there would always be a prefix character (the space),
it decremented "len" unconditionally, thus passing len=0 to emit_line,
which would then blindly call emit_line_0 with len=-1 which would
pass that value on to fwrite as SIZE_MAX.  Boom.

Signed-off-by: Jim Meyering <redacted>
---
 diff.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/diff.c b/diff.c
index ba5f7aa..57eea74 100644
--- a/diff.c
+++ b/diff.c

@@ -1117,8 +1117,13 @@ static void fn_out_consume(void *priv, char *line, unsigned long len)
 			emit_line(ecbdata->opt, plain, reset, line, len);
 			fputs("~\n", ecbdata->opt->file);
 		} else {
-			/* don't print the prefix character */
-			emit_line(ecbdata->opt, plain, reset, line+1, len-1);
+			/* If there is a prefix character, skip it.
+			   With diff_suppress_blank_empty, there may be none. */
+			if (line[0] != '\n') {
+			      line++;
+			      len--;
+			}
+			emit_line(ecbdata->opt, plain, reset, line, len);
 		}
 		return;
 	}
--

1.7.5.2.316.gbcebc8b