coprocess: enhance error handling in ObjectPostProcess (#2638)
coprocess: enhance error handling in ObjectPostProcess (#2638)

This small tweak could be useful in scenarios where the request URL is modified by a plugin (and the URL is invalid or empty), for example:

```python
from tyk.decorators import *
print("middleware.py is loaded")
@Hook
def myhook(request, session, spec):
  print("myhook called")
  request.object.url = ''
  return request, session
```
In the current implementation, when `ParseRequestURI` fails, the following line panics because of a nil `r.URL`:
```golang
r.URL.RawQuery = values.Encode()
```
lonelycode/tyk,lonelycode/tyk,lonelycode/tyk