This was causing dlsym to return the pointer to the wrapper's function when
SwiftShader didn't support an entry point.

Bug 25152138

Change-Id: I92d1c9258c823acf0dc60dca9dc3d7dd552512b3
Reviewed-on: https://swiftshader-review.googlesource.com/4120Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Tested-by: Greg Hartman <ghartman@google.com>