Added empty AsmBlock check to sanity_check

It causes ambiguities during assembling and there's no straightforward warning

miasm/core/asmblock.py

@@ -729,6 +729,10 @@ def sanity_check(self):
         for loc_key in self._nodes:
             if loc_key not in self._loc_key_to_block:
                 raise RuntimeError("Not supported yet: every node must have a corresponding AsmBlock")
+
+            if not self.loc_key_to_block(loc_key).lines:
+                raise RuntimeError("Blocks cannot be empty")
+
             # No next constraint to self
             if (loc_key, loc_key) in next_edges:
                 raise RuntimeError('Bad constraint: self in next')