oinspect.find_file: Additional safety if file cannot be found.

In some code paths, e.g. when introspecting fortran objects generated by
f2py, oinspect.find_file will raise an UnboundLocalError.  Prevent this by
setting fname to None.

IPython/core/oinspect.py

@@ -250,6 +250,7 b' def find_file(obj):'
     if hasattr(obj, '__wrapped__'):
         obj = obj.__wrapped__
     
+    fname = None
     try:
         fname = inspect.getabsfile(obj)
     except TypeError:
@@ -260,9 +261,9 b' def find_file(obj):'
                 fname = inspect.getabsfile(obj.__class__)
             except TypeError:
                 # Can happen for builtins
-                fname = None
+                pass
     except:
-        fname = None
+        pass
     return fname