C++: Fix strncpy model.

cpp/ql/lib/semmle/code/cpp/models/implementations/Strcpy.qll

@@ -108,7 +108,7 @@ class StrcpyFunction extends ArrayFunction, DataFlowFunction, TaintFunction, Sid
     // these may do only a partial copy of the input buffer to the output
     // buffer
     exists(this.getParamSize()) and
-    input.isParameter(this.getParamSrc()) and
+    input.isParameterDeref(this.getParamSrc()) and
     (
       output.isParameterDeref(this.getParamDest()) or
       output.isReturnValueDeref()

cpp/ql/test/library-tests/dataflow/taint-tests/taint.cpp

@@ -709,5 +709,5 @@ char * strncpy (char *, const char *, unsigned long);
 void test_strncpy(char* d, char* s) {
 	argument_source(s);
 	strncpy(d, s, 16);
-	sink(d); // $ ast MISSING: ir
+	sink(d); // $ ast ir
 }