release: Add missing return to _check_required_env_var()

Otherwise none of the calls to this function will actually continue
after it's called.

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

tools/packaging/release/release.sh

@@ -45,6 +45,8 @@ function _check_required_env_var()
 
 	[ -z "${env_var}" ] && \
 		_die "\"${1}\" environment variable is required but was not set"
+
+	return 0
 }
 
 function _next_release_version()