Fix out of boundary read.

If a binary file which starts with 0x00 is edited, xrdb performs
an off-by-one read outside of its buffer.

The fix is simple: Do not try to append an empty string to buffer,
which would be a no-op anyway.

Proof of Concept (compile with -fsanitize=address):

$ dd if=/dev/zero bs=1 count=1 of=poc.txt
$ xrdb -edit poc.txt

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/xrdb.c b/xrdb.c
index 6054790..3f6e533 100644
--- a/xrdb.c
+++ b/xrdb.c
@@ -745,6 +745,8 @@ EditFile(Entries *new, FILE *in, FILE *out)
             buff[0] = '\0';
             if (!fgets(buff, BUFSIZ, in))
                 goto cleanup;
+            if (buff[0] == '\0')
+                continue;
             AppendToBuffer(&b, buff, strlen(buff));
             c = &b.buff[b.used - 1];
             if ((*(c--) == '\n') && (b.used == 1 || *c != '\\'))