Renaming Labels Exception

Set a label at an instruction somewhere in the middle of a block (hotkey=`n`). When the label is set, the relevant `QBlock.addr_to_labels` is never set so if you rename that label, you'll get this:

```
Traceback (most recent call last):
File "/home/steve/code/angr-dev/angr-management/angrmanagement/ui/dialogs/rename_label.py", line 114, in _on_ok_clicked
self._disasm_view.rename_label(self._label_addr, label)
File "/home/steve/code/angr-dev/angr-management/angrmanagement/ui/views/disassembly_view.py", line 356, in rename_label
self._flow_graph.update_label(addr, is_renaming=is_renaming)
File "/home/steve/code/angr-dev/angr-management/angrmanagement/ui/widgets/qgraph.py", line 129, in update_label
block.update_label(label_addr)
File "/home/steve/code/angr-dev/angr-management/angrmanagement/ui/widgets/qblock.py", line 85, in update_label
raise Exception('Label at address %#x is not found.' % label_addr)
Exception: Label at address 0x4007a9 is not found.
```

I'm guessing this was originally intended for setting a label for a block at the block's start address but the hotkey works for any instruction address (as it should, like IDA). Either way, the label is never actually shown and you can't rename it like above.