tdf#128447 sc/vml: accept <? ?> xml prolog

... or processing instruction, or text directive.

The inability to parse this was exposed in LO 6.2
with commit 2cae2ecfef

I added <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
to the top of vmlDrawing1.vml from the unit test,
which invalidates the entire .vml file and the two
controls are lost without this fix.

Change-Id: Ia1e6d5f9bb932756f70dca59fa8b32004c9e8013
Reviewed-on: https://gerrit.libreoffice.org/81730
Reviewed-by: Michael Stahl <michael.stahl@cib.de>
Tested-by: Jenkins

oox/source/vml/vmlinputstream.cxx

@@ -169,6 +169,12 @@ void lclProcessElement( OStringBuffer& rBuffer, const OString& rElement )
         // do nothing
     }
 
+    // just append any xml prolog (text directive) or processing instructions: <?...?>
+    else if( (nElementLen >= 4) && (pcOpen[ 1 ] == '?') && (pcClose[ -1 ] == '?') )
+    {
+        rBuffer.append( rElement );
+    }
+
     // replace '<br>' element with newline
     else if( (nElementLen >= 4) && (pcOpen[ 1 ] == 'b') && (pcOpen[ 2 ] == 'r') && (lclFindNonWhiteSpace( pcOpen + 3, pcClose ) == pcClose) )
     {