Fix integer overflow in PMF video frame buffer allocation

pmf_init reported stream dimensions without a cap, and PMFView::Draw
allocated width * height * 4 with 32-bit int arithmetic, so a crafted
ICON1.PMF could overflow the allocation to a small buffer while
sws_scale wrote the full frame.

- Reject videos with dimensions outside 1..720x480 in pmf_init (PMFs on
  the PSP never exceed 720x480).
- Use size_t arithmetic for the frame buffer allocation.

UI/GameScreen.cpp

@@ -113,7 +113,9 @@ public:
 
 		Draw::DrawContext *draw = dc.GetDrawContext();
 
-		std::vector<u8> frame(width_ * height_ * 4);
+		// Dimensions are capped by pmf_init, but use size_t arithmetic anyway
+		// so a regression can't overflow the allocation.
+		std::vector<u8> frame((size_t)width_ * (size_t)height_ * 4);
 		if (pmf_update(player_, startTime_.ElapsedSeconds(), frame.data())) {
 			if (curFrame_) {
 				curFrame_->Release();