build: silence clang false positive

Clang couldn't quite see that the same condition of
(flags & VIR_DOMAIN_MEM_CONFIG) is used twice, such that
the second block is guaranteed that def was assigned in
the first block.

* src/libxl/libxl_driver.c (libxlDomainSetMemoryFlags): Add a hint
for clang.

src/libxl/libxl_driver.c

@@ -1535,6 +1535,8 @@ libxlDomainSetMemoryFlags(virDomainPtr dom, unsigned long memory,
     ret = 0;
 
     if (flags & VIR_DOMAIN_MEM_CONFIG) {
+        /* clang 2.9 needs an extra hint.  */
+        sa_assert(def);
         def->mem.cur_balloon = memory;
         ret = virDomainSaveConfig(driver->configDir, def);
     }