fix bug in SbxArray::Merge

bug was recently introduced by
commit a5c23283
"BASIC: Store directly SbxVarEntry in SbxArray"

bug was found by clang-tidy

Change-Id: Ibcce6d6024ac60eb143cd873760586a7f6231858
Reviewed-on: https://gerrit.libreoffice.org/23890Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

basic/source/sbx/sbxarray.cxx

@@ -343,7 +343,7 @@ void SbxArray::Merge( SbxArray* p )
             {
                 // Take this element and clear the original.
                 rEntry2.mpVar = rEntry1.mpVar;
-                rEntry2.mpVar.Clear();
+                rEntry1.mpVar.Clear();
                 break;
             }
         }