testsuite: add test for is_valid_type.

src/python/espressomd/particle_data.pyx

@@ -151,6 +151,8 @@ cdef class ParticleHandle:
 
         def __set__(self, _pos):
             cdef double mypos[3]
+            if np.isnan(_pos).any() or np.isinf(_pos).any():
+                raise ValueError("invalid particle position")
             check_type_or_throw_except(
                 _pos, 3, float, "Position must be 3 floats")
             for i in range(3):

src/python/espressomd/utils.pyx

@@ -284,7 +284,8 @@ def is_valid_type(value, t):
     Extended checks for numpy int and float types.
 
     """
-
+    if value is None:
+        return False
     if t == int:
         return isinstance(value, (int, np.integer, np.long))
     elif t == float: