hw/display/bcm2835_fb: Silence Coverity warning about multiply overflow

Coverity complains (CID 1395628) that the multiply in the calculation
of the framebuffer base is performed as 32x32 but then used in a
context that takes a 64-bit hwaddr. This can't actually ever
overflow the 32-bit result, because of the constraints placed on
the s->config values in bcm2835_fb_validate_config(). But we
can placate Coverity anyway, by explicitly casting one of the
inputs to a hwaddr, so the whole expression is calculated with
64-bit arithmetic.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20181005133012.26490-1-peter.maydell@linaro.org

hw/display/bcm2835_fb.c

@@ -190,7 +190,7 @@ static void fb_update_display(void *opaque)
     }
 
     if (s->invalidate) {
-        hwaddr base = s->config.base + xoff + yoff * src_width;
+        hwaddr base = s->config.base + xoff + (hwaddr)yoff * src_width;
         framebuffer_update_memory_section(&s->fbsection, s->dma_mr,
                                           base,
                                           s->config.yres, src_width);