Check that visit ref exists before adding to list

The situation can occur where a given visit specified in the list
of selected visits is missing from the inputWarpDict for a given
tract/patch dataRef.  Thus, check that the visit actually exists
in the inputWarpDict before trying to append it to the filtered
inputs list.

python/lsst/pipe/tasks/assembleCoadd.py

@@ -1332,7 +1332,8 @@ def filterWarps(self, inputs, goodVisits):
         inputWarpDict = {inputRef.ref.dataId['visit']: inputRef for inputRef in inputs}
         filteredInputs = []
         for visit in goodVisits.keys():
-            filteredInputs.append(inputWarpDict[visit])
+            if visit in inputWarpDict:
+                filteredInputs.append(inputWarpDict[visit])
         return filteredInputs