msg: return zero length if bstr_split_utf8 fails

In theory bstr_split_utf8 should skip invalid sequence and move further,
but it doesn't do that currently, so just the string if unsuported code if
found.

Fixes infinite loop on code.len == 0 condition.

Fixes: 5864b72d1a0aecd7e2d56e546fb615d291c88274