common: spice_memdup could accept NULL

(this patch is not to solve a crash fix, but to align with glib API)

common/mem.c

@@ -74,6 +74,10 @@ void *spice_memdup(const void *mem, size_t n_bytes)
 {
     void *copy;
 
+    if (mem == NULL) {
+        return NULL;
+    }
+
     copy = spice_malloc(n_bytes);
     memcpy(copy, mem, n_bytes);
     return copy;