exec.c: fix setting 1-byte-long watchpoints

With commit 05068c0dfb5b 'exec.c: Relax restrictions on watchpoint length
and alignment' it's no longer possible to set 1-byte-long watchpoint
because of incorrect address range check.
Fix that by changing condition that checks for address wraparound.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1411016616-29879-1-git-send-email-jcmvbkbc@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/exec.c b/exec.c
index 2b24651..759055d 100644
--- a/exec.c
+++ b/exec.c
@@ -595,7 +595,7 @@
     CPUWatchpoint *wp;
 
     /* forbid ranges which are empty or run off the end of the address space */
-    if (len == 0 || (addr + len - 1) <= addr) {
+    if (len == 0 || (addr + len - 1) < addr) {
         error_report("tried to set invalid watchpoint at %"
                      VADDR_PRIx ", len=%" VADDR_PRIu, addr, len);
         return -EINVAL;