BUG: adds check in dtype.name that string starts with "numpy." prefix before removing it (Fixes #4357)

This is my second contribution to NumPy; any and all feedback is welcome.

Fixes #4357. Adds check in ``arraydescr_typename_get()`` (implementation of dtype.name) that the name string inherited from  ``typeobj->tp_name`` does indeed start with "numpy." before offsetting the pointer to remove it.