lm32: milkymist-tmu2: fix a third integer overflow

Don't truncate the multiplication and do a 64 bit one instead
because the result is stored in a 64 bit variable.

This fixes a similar coverity warning to commits 237a865 and
4382fa6, in a similar way, and is the final third of the fix for
coverity CID 1167561 (hopefully!).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Michael Walle <michael@walle.cc>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/display/milkymist-tmu2.c

@@ -293,7 +293,7 @@ static void tmu2_start(MilkymistTMU2State *s)
     cpu_physical_memory_unmap(mesh, mesh_len, 0, mesh_len);
 
     /* Write back the OpenGL framebuffer to the QEMU framebuffer */
-    fb_len = 2 * s->regs[R_DSTHRES] * s->regs[R_DSTVRES];
+    fb_len = 2ULL * s->regs[R_DSTHRES] * s->regs[R_DSTVRES];
     fb = cpu_physical_memory_map(s->regs[R_DSTFBUF], &fb_len, 1);
     if (fb == NULL) {
         glDeleteTextures(1, &texture);