Rewrite the logic in VRTSourcedRasterBand::SetMetadata.

CID 1338309 (#1 of 1): Structurally dead code (UNREACHABLE)
unreachable: Since the loop increment i++; is unreachable, the loop
body will never execute more than once

By testing if poSource is a nullptr first and returning an error if it
is a nullptr, I think the logic is much easier to follow.  I misread 
the logic here:

https://trac.osgeo.org/gdal/browser/trunk/gdal/frmts/vrt/vrtsourcedrasterband.cpp?rev=31122#L1400

and created r31355 that got it wrong.


git-svn-id: https://svn.osgeo.org/gdal/trunk@31361 f0d54148-0727-0410-94bb-9a71ac55c965

gdal/frmts/vrt/vrtsourcedrasterband.cpp

@@ -1380,14 +1380,12 @@ CPLErr VRTSourcedRasterBand::SetMetadata( char **papszNewMD, const char *pszDoma
             VRTSource *poSource = poDriver->ParseSource( psTree, NULL );
             CPLDestroyXMLNode( psTree );
 
-            if( poSource != NULL )
-            {
-                CPLErr eErr = AddSource( poSource );
-                if( eErr != CE_None )
-                    return eErr;
-            }
+            if( poSource == NULL )
+                return CE_Failure;
 
-            return CE_Failure;
+            CPLErr eErr = AddSource( poSource );
+            if( eErr != CE_None )
+                return eErr;
         }
 
         return CE_None;