glib-aux: assert that nm_utils_to_string_buffer_init() does not use the global buffer

For convenience, most to-string methods call nm_utils_to_string_buffer_init().
This allows to omit the string buffer and use a global (thread-local)
buffer.

That "convenience" seems error prone. Start drop it.

Start by adding a g_return_if_reached() assertion to catch the cases
that use it.

src/libnm-glib-aux/nm-shared-utils.c

@@ -871,6 +871,11 @@ nm_utils_to_string_buffer_init(char **buf, gsize *len)
     if (!*buf) {
         *buf = _nm_utils_to_string_buffer;
         *len = NM_UTILS_TO_STRING_BUFFER_SIZE;
+
+        /* We no longer want to support callers to omit the buffer
+         * and fallback to the global buffer. Callers should be fixed
+         * to always provide a valid buffer. */
+        g_return_if_reached();
     }
 }