Check for linenum being None in Node.warn

This might happen when emitting a warning on PyPy

Author: nicoddemus

_pytest/main.py

@@ -310,7 +310,10 @@ def warn(self, code, message):
         if fslocation is None:
             fslocation = getattr(self, "fspath", None)
         else:
-            fslocation = "%s:%s" % (fslocation[0], fslocation[1] + 1)
+            path, linenum = fslocation
+            if linenum is not None:
+                linenum += 1
+            fslocation = "%s:%s" % (path, linenum)
 
         self.ihook.pytest_logwarning.call_historic(kwargs=dict(
             code=code, message=message,