avoid crash with incomplete font definitions ( IcoMoon )

src/graphic/QskGlyphTable.cpp

@@ -50,6 +50,8 @@ namespace PostTableParser
             const auto from = reinterpret_cast< const uint8_t* >( glyphData + nglyphs );
             const auto to = reinterpret_cast< const uint8_t* >( blob.data() + blob.size() );
 
+            if ( to > from )
+            {
                 for ( auto s = from; s < to; s += *s + 1 )
                     strings += s;
 
@@ -57,10 +59,11 @@ namespace PostTableParser
                 {
                     const int idx = qFromBigEndian( glyphData[i] ) - 258;
 
-                if ( idx >= 0 )
+                    if ( idx >= 0 && idx < strings.size() )
                         names.insert( toString( strings[idx] ), i );
                 }
             }
+        }
 
         return names;
     }