Fix regression in embed() from pull-request #2096.

With certain sets of arguments `compile_flags` might be left as `None`. This
caused IPython to internally raise a TypeError when it tried to do a
bitwise or between `shell.compile.flags` and `PyCF_ONLY_AST` in
`CachingCompiler.ast_parse`.

The regression was introduced in:
  b70ac12 embed(): Default to the future compile flags of the calling frame.

IPython/frontend/terminal/embed.py

@@ -200,7 +200,8 b' class InteractiveShellEmbed(TerminalInteractiveShell):'
             module.__dict__ = global_ns
 
         # Get locals and globals from caller
-        if (local_ns is None or module is None) and self.default_user_namespaces:
+        if ((local_ns is None or module is None or compile_flags is None)
+            and self.default_user_namespaces):
             call_frame = sys._getframe(stack_depth).f_back
 
             if local_ns is None:
@@ -233,7 +234,8 b' class InteractiveShellEmbed(TerminalInteractiveShell):'
             self.init_user_ns()
 
         # Compiler flags
-        self.compile.flags = compile_flags
+        if compile_flags is not None:
+            self.compile.flags = compile_flags
 
         # Patch for global embedding to make sure that things don't overwrite
         # user globals accidentally. Thanks to Richard <[e-mail]>