HPCC-17051 Coverity: Null dereference warning in cassandrawu.cpp

Add an assert to validate parameters passed in are consistent.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>

plugins/cassandra/cassandrawu.cpp

@@ -3391,6 +3391,7 @@ public:
                 {
                 case WUSFappvalue:
                 {
+                    assertex(fv);
                     const char *name = fv;
                     fv = fv + strlen(fv)+1;
                     if (isWild)