[svg] Clear correct flags for doc ownership

This issue was discovered with an SVG based font with some documents
compressed and other uncompressed. After loading the first compressed
document the ownership flag on the glyph slot was set to true but never
set to false. As a result after loading a compressed document a glyph
from an uncompressed document would load fine, but when this glyph slot
was cleared it would try to free its document resulting in a wild free.

* src/base/ftobjs.c (ft_glyphslot_clear): clear correct flags

Fixes: #1162

src/base/ftobjs.c

@@ -605,7 +605,7 @@
 
 
         FT_FREE( doc->svg_document );
-        slot->internal->load_flags &= ~FT_GLYPH_OWN_GZIP_SVG;
+        slot->internal->flags &= ~FT_GLYPH_OWN_GZIP_SVG;
       }
     }
 #endif