QWebpHandler: add a missing std::move() in read()

Coverity complained that we could have moved `frame` into *image, but
copied.

It's correct, but the assignment is far away from the end of the scope
that defines the lifetime of the moved-from object, so just adding
std::move() here would impair readability.

We can, however, switch the polarity of the guarding `if` so that the
animating branch comes first, which must always copy, since it retains
the old image to apply the diff to the next frame onto, and then the
else branch _is_ near the end of the scope, and we _can_ add the
std::move().

Make it so.

Amends 1d4f24820c0fff474d524e006d715e13e409a4b8 (5.7}, but not picking
back far, since this is just a tiny "missed optimization".

Pick-to: 6.12
Coverity-Id: 426840
Change-Id: I1949d71ce77d5dbf4ca8a675686c9767ac492f90
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>