fix bug where it assumed the flags array has a shape big enough to se…

…t all bits (reported by Amy Rankine)

python/sdss_semaphore/__init__.py

@@ -202,7 +202,10 @@ def is_attribute_set(self, key, value) -> np.array:
         if len(bits) == 0:
             raise ValueError(f"No bits found with attribute {key}={value}")
         num, offset = np.divmod(bits, self.n_bits)
-        return np.any(self.array[:, num] & (1 << offset), axis=1)
+        N, B = self.array.shape
+        can_be_set = B > num
+        return np.any(self.array[:, num[can_be_set]] & (1 << offset[can_be_set]), axis=1)
+
 
     def get_bits_with_attribute(self, key, value) -> List[int]:
         """
@@ -342,7 +345,4 @@ def __repr__(self):
 
     def __len__(self):
         N, B = self.array.shape
-        return N
-
-
-
+        return N