bmp failure not detected as such because of svg fallback test

I thought it was suspicious that everything was a pass

Change-Id: I10b98cc19d2e23bd5d518113785aeca3a0ea346a
Reviewed-on: https://gerrit.libreoffice.org/18662Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/gdi/impgraph.cxx

@@ -1527,6 +1527,7 @@ SvStream& ReadImpGraphic( SvStream& rIStm, ImpGraphic& rImpGraphic )
                     }
                     else
                     {
+                        sal_uInt32 nOrigError = rIStm.GetErrorCode();
                         // try to stream in Svg defining data (length, byte array and evtl. path)
                         // See below (operator<<) for more information
                         const sal_uInt32 nSvgMagic((sal_uInt32('s') << 24) | (sal_uInt32('v') << 16) | (sal_uInt32('g') << 8) | sal_uInt32('0'));
@@ -1535,7 +1536,11 @@ SvStream& ReadImpGraphic( SvStream& rIStm, ImpGraphic& rImpGraphic )
                         rIStm.ResetError();
                         rIStm.ReadUInt32( nMagic );
 
-                        if(nSvgMagic == nMagic)
+                        if (nSvgMagic != nMagic)
+                        {
+                            rIStm.SetError(nOrigError);
+                        }
+                        else
                         {
                             sal_uInt32 mnSvgDataArrayLength(0);
                             rIStm.ReadUInt32( mnSvgDataArrayLength );