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>

trailer.c

@@ -507,6 +507,8 @@ static int git_trailer_default_config(const char *conf_key, const char *value,
 				warning(_("unknown value '%s' for key '%s'"),
 					value, conf_key);
 		} else if (!strcmp(trailer_item, "separators")) {
+			if (!value)
+				return config_error_nonbool(conf_key);
 			separators = xstrdup(value);
 		}
 	}