lm32: milkymist-tmu2: fix another 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 commit 237a865,
in a similar way, and is the other half of the fix for
coverity CID 1167561.

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

hw/display/milkymist-tmu2.c

@@ -257,7 +257,7 @@ static void tmu2_start(MilkymistTMU2State *s)
     glColor4f(m, m, m, (float)(s->regs[R_ALPHA] + 1) / 64.0f);
 
     /* Read the QEMU dest. framebuffer into the OpenGL 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, 0);
     if (fb == NULL) {
         glDeleteTextures(1, &texture);