Fix mismatching delete in IFF image handler code

Use delete[] for a buffer allocated with new[].

Closes https://github.com/wxWidgets/wxWidgets/pull/1684

src/common/imagiff.cpp

@@ -414,7 +414,7 @@ int wxIFFDecoder::ReadIFF()
         const byte *cmapptr = dataptr + 8;
         colors = chunkLen / 3;                  // calc no of colors
 
-        wxDELETE(m_image->pal);
+        wxDELETEA(m_image->pal);
         m_image->colors = colors;
         if (colors > 0) {
         m_image->pal = new byte[3*colors];