small spice_strdup optimization

avoid to compute the string length twice and use memcpy instead of
strcpy which is faster not having to check for terminator.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Jonathon Jongsma <jjongsma@redhat.com>

common/mem.c

@@ -46,13 +46,15 @@ size_t spice_strnlen(const char *str, size_t max_len)
 char *spice_strdup(const char *str)
 {
     char *copy;
+    size_t len;
 
     if (str == NULL) {
         return NULL;
     }
 
-    copy = (char *)spice_malloc(strlen(str) + 1);
-    strcpy(copy, str);
+    len = strlen(str) + 1;
+    copy = (char *)spice_malloc(len);
+    memcpy(copy, str, len);
     return copy;
 }