Check that we parsed something in wxBitmapBundle::FromSVG()

Don't always return success from this function, NanoSVG just skips
everything until the start of the XML prologue and doesn't return an
error even if it doesn't find it at all, so check that it could parse at
least something to avoid returning a "valid" bundle not containing
anything at all.

Add a unit test checking that we actually can't create an SVG from a
.bmp file (which is something that "worked" before).

tests/graphics/bmpbundle.cpp

@@ -140,9 +140,13 @@ TEST_CASE("BitmapBundle::FromSVG", "[bmpbundle][svg]")
 
 TEST_CASE("BitmapBundle::FromSVGFile", "[bmpbundle][svg][file]")
 {
-    wxBitmapBundle b = wxBitmapBundle::FromSVGFile("horse.svg", wxSize(20, 20));
+    const wxSize size(20, 20); // completely arbitrary
+
+    CHECK( !wxBitmapBundle::FromSVGFile("horse.bmp", size).IsOk() );
+
+    wxBitmapBundle b = wxBitmapBundle::FromSVGFile("horse.svg", size);
     REQUIRE( b.IsOk() );
-    CHECK( b.GetDefaultSize() == wxSize(20, 20) );
+    CHECK( b.GetDefaultSize() == size );
 }
 
 #endif // wxHAS_SVG