COV: Fixing coverity issue 1266521 (Wrapper object use after free)

Testing/Code/Hyperspectral/otbSparseUnmixingImageFilter.cxx

@@ -59,7 +59,7 @@ int otbSparseUnmixingImageFilterTest ( int argc, char * argv[] )
   std::string inputImageName [ nbInputImages ];
   for ( unsigned int i = 0; i < nbInputImages; i++ )
     inputImageName[i] = parseResult->GetParameterString("--InputImages", i);
-  const char * outputImageName = parseResult->GetParameterString("--OutputImages").c_str();
+  std::string outputImageName = parseResult->GetParameterString("--OutputImages");
   const double threshold = parseResult->IsOptionPresent("--Threshold") ?
     parseResult->GetParameterDouble("--Threshold") : 10.;