migration: fix xbzrle encoding rate calculation

It's reported an error of implicit conversion from "unsigned long" to
"double" when compiling with Clang 10. Simply make the encoding rate 0
when the encoded_size is 0.

Fixes: e460a4b
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200617201309.1640952-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

migration/ram.c

@@ -913,10 +913,8 @@ static void migration_update_rates(RAMState *rs, int64_t end_time)
         unencoded_size = (xbzrle_counters.pages - rs->xbzrle_pages_prev) *
                          TARGET_PAGE_SIZE;
         encoded_size = xbzrle_counters.bytes - rs->xbzrle_bytes_prev;
-        if (xbzrle_counters.pages == rs->xbzrle_pages_prev) {
+        if (xbzrle_counters.pages == rs->xbzrle_pages_prev || !encoded_size) {
             xbzrle_counters.encoding_rate = 0;
-        } else if (!encoded_size) {
-            xbzrle_counters.encoding_rate = UINT64_MAX;
         } else {
             xbzrle_counters.encoding_rate = unencoded_size / encoded_size;
         }