[PATCH] strbuf: stop out-of-boundary warnings from Coverity

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

It usually goes like this

    strbuf sb = STRBUF_INIT;
    if (!strncmp(sb.buf, "foo", 3))
       printf("%s", sb.buf + 3);

Coverity thinks that printf() can be executed, and because initial
sb.buf only has one character (from strbuf_slopbuf), sb.buf + 3 is out
of bound. What it does not recognize is strbuf_slopbuf[0] is always (*)
zero. We always do some string comparison before jumping ahead to
"sb.buf + 3" and those operations will stop out of bound accesses.

Just make strbuf_slopbuf[] large enough to keep Coverity happy. If it's
happy, we'll have cleaner defect list and better chances of spotting
true defects.

(*) It's not entirely wrong though. Somebody may do sb.buf[0] = 'f'
    right after variable declaration and ruin all unused strbuf.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 There are lots of false warnings like this from Coverity. I just
 wanted to kill them off so we can spot more serious problems easier.
 I can't really verify that this patch shuts off those warnings
 because scan.coverity.com policy does not allow forks.

 I had another patch that avoids corrupting strbuf_slopbuf, by putting
 it to .rodata section. The patch is more invasive though, because
 this statement buf.buf[buf.len] = '\0' now has to make sure buf.buf
 is not strbuf_slopbuf. It feels safer, but probably not enough to
 justify the change.

 strbuf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/strbuf.c b/strbuf.c
index 0d4f4e5..0d7c3cf 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -16,7 +16,12 @@ int starts_with(const char *str, const char *prefix)
  * buf is non NULL and ->buf is NUL terminated even for a freshly
  * initialized strbuf.
  */
+#ifndef __COVERITY__
 char strbuf_slopbuf[1];
+#else
+/* Stop so many incorrect out-of-boundary warnings from Coverity */
+char strbuf_slopbuf[64];
+#endif
 
 void strbuf_init(struct strbuf *sb, size_t hint)
 {
-- 
2.3.0.rc1.137.g477eb31