Fix: crash loading & saving large Iris images

Integer overflow in IMB_convert_rgba_to_abgr assigned a negative value
to size_t resulting in a large value which wrote past the buffer bounds.

Back ported from: d256b7f6882f1d3d6162392c81e2b3c0e62cd1a9

Pull Request: https://projects.blender.org/blender/blender/pulls/137447

source/blender/imbuf/intern/imageprocess.cc

@@ -24,7 +24,7 @@ void IMB_convert_rgba_to_abgr(ImBuf *ibuf)
   float rtf, *cpf = ibuf->float_buffer.data;
 
   if (ibuf->byte_buffer.data) {
-    size = ibuf->x * ibuf->y;
+    size = size_t(ibuf->x) * size_t(ibuf->y);
 
     while (size-- > 0) {
       rt = cp[0];
@@ -38,7 +38,7 @@ void IMB_convert_rgba_to_abgr(ImBuf *ibuf)
   }
 
   if (ibuf->float_buffer.data) {
-    size = ibuf->x * ibuf->y;
+    size = size_t(ibuf->x) * size_t(ibuf->y);
 
     while (size-- > 0) {
       rtf = cpf[0];