Fix a crash in nested file functions

Summary: UrlFile functions correctly chased through multiple actrecs
to find the callsite. ExecutionContext::setVar only went one deep.

The result was that the example crashed because it tried to set a
variable in a frame that didn't have a VarEnv.

Reviewed By: @swtaarrs

Differential Revision: D1614350
Merge pull request #4033 from markw65/HHVM-3.3

Fix a crash in nested file functions