config_cache: check return value of `git_config__lookup_entry`

Callers of `git_config__cvar` already handle the case where the
function returns an error due to a failed configuration variable
lookup, but we are actually swallowing errors when calling
`git_config__lookup_entry` inside of the function.

Fix this by returning early when `git_config__lookup_entry`
returns an error. As we call `git_config__lookup_entry` with
`no_errors == false` which leads us to call `get_entry` with
`GET_NO_MISSING` we will not return early when the lookup fails
due to a missing entry. Like this we are still able to set the
default value of the cvar and exit successfully.

src/config_cache.c

@@ -86,7 +86,8 @@ int git_config__cvar(int *out, git_config *config, git_cvar_cached cvar)
 	struct map_data *data = &_cvar_maps[(int)cvar];
 	git_config_entry *entry;
 
-	git_config__lookup_entry(&entry, config, data->cvar_name, false);
+	if ((error = git_config__lookup_entry(&entry, config, data->cvar_name, false)) < 0)
+		return error;
 
 	if (!entry)
 		*out = data->default_value;