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>

mailinfo.c

@@ -1253,6 +1253,8 @@ static int git_mailinfo_config(const char *var, const char *value,
 		return 0;
 	}
 	if (!strcmp(var, "mailinfo.quotedcr")) {
+		if (!value)
+			return config_error_nonbool(var);
 		if (mailinfo_parse_quoted_cr_action(value, &mi->quoted_cr) != 0)
 			return error(_("bad action '%s' for '%s'"), value, var);
 		return 0;