Fix build warning: uninitialized variable xtype

src/pnetcdf/_Variable.c: In function '__pyx_pw_7pnetcdf_9_Variable_8Variable_1__init__':
src/pnetcdf/_Variable.c:10050:11: warning: '__pyx_v_xtype' may be used uninitialized in this function [-Wmaybe-uninitialized]
   nc_type __pyx_v_xtype;
           ^~~~~~~~~~~~~

Author: wkliao

src/pnetcdf/_Variable.pyx

@@ -111,8 +111,10 @@ cdef class Variable:
                 xtype = _nptonctype[nc_dtype.str[1:]]
             else:
                 raise TypeError('illegal data type, must be one of %s, got %s' % (_supportedtypes, nc_dtype.str[1:]))
-        if isinstance(nc_dtype, int):
+        elif isinstance(nc_dtype, int):
             xtype = nc_dtype
+        else:
+            raise TypeError('illegal data type, must be an int, got %s' % (nc_dtype.str[1:]))
         self.xtype = xtype
         self.dtype = np.dtype(_nctonptype[xtype])