This patch reduces three sets of warnings
when compiling on Windows:
* In gimp-layer-modes.c, we move the
g_return_val_if_reached () in
gimp_layer_mode_get_format () to the
default case in the last switch statement
to prevent a warning about an unhandled
switch case
* In spline.c, we initialize V[] to prevent
a warning about uninitialized values
* In flame.c, we unconditionally call
g_bytes_unref () to prevent a warning
about dangling pointers.
