Set #VALUE! error in COVAR() OpenCL implementation when appropriate

Returning -DBL_MAX doesn't make sense. The traditional C++ implementation and
other spreadsheet products return an error.

Change-Id: I71117c0b1f113d31ad6eb0c1798b74f96b3a75de

sc/source/core/opencl/op_statistical.cxx

@@ -7048,7 +7048,7 @@ void OpCovar::GenSlidingWindowFunction(std::stringstream& ss,
                 ss << "    }\n";
             }
             ss << "    if(cnt < 1) {\n";
-            ss << "        return -DBL_MAX;\n";
+            ss << "        return CreateDoubleError(errNoValue);\n";
             ss << "    }\n";
             ss << "    else {\n";
             ss << "        vMean0 = vSum0 / cnt;\n";