Don't assume that SyntaxTB is always called with a SyntaxError

Closes gh-4361

IPython/core/ultratb.py

@@ -1202,7 +1202,8 b' class SyntaxTB(ListTB):'
         # If the source file has been edited, the line in the syntax error can
         # be wrong (retrieved from an outdated cache). This replaces it with
         # the current value.
-        if isinstance(value.filename, py3compat.string_types) \
+        if isinstance(value, SyntaxError) \
+                and isinstance(value.filename, py3compat.string_types) \
                 and isinstance(value.lineno, int):
             linecache.checkcache(value.filename)
             newtext = ulinecache.getline(value.filename, value.lineno)