config: handle NULL value when parsing non-bools

When the config parser sees an "implicit" bool like:

  [core]
  someVariable

it passes NULL to the config callback. Any callback code which expects a
string must check for NULL. This usually happens via helpers like
git_config_string(), etc, but some custom code forgets to do so and will
segfault.

These are all fairly vanilla cases where the solution is just the usual
pattern of:

  if (!value)
        return config_error_nonbool(var);

though note that in a few cases we have to split initializers like:

  int some_var = initializer();

into:

  int some_var;
  if (!value)
        return config_error_nonbool(var);
  some_var = initializer();

There are still some broken instances after this patch, which I'll
address on their own in individual patches after this one.

Reported-by: Carlos Andrés Ramírez Cataño <antaigroupltda@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes-utils.c

@@ -112,6 +112,8 @@ static int notes_rewrite_config(const char *k, const char *v,
 		}
 		return 0;
 	} else if (!c->refs_from_env && !strcmp(k, "notes.rewriteref")) {
+		if (!v)
+			return config_error_nonbool(k);
 		/* note that a refs/ prefix is implied in the
 		 * underlying for_each_glob_ref */
 		if (starts_with(v, "refs/notes/"))