Extend STR_NULL and str_init() to set a type explicitly. This

allows those macros to be used elsewhere in the function body,
not only in variable declaration(s), i.e.:

void foobar(int baz)
{
    str localstr;

    if (random() == 42) {
       localstr = str_init(I_STR LUCKY_STR FIN_STR);
    } else {
        localstr = str_init(I_STR UNLUCKY_STR FIN_STR);
    }
    some_api(&localstr);
}

str.h

@@ -59,8 +59,8 @@ struct __str {
 typedef struct __str str;
 
 /* str initialization */
-#define STR_NULL {NULL, 0}
-#define str_init(_string)  {_string, sizeof(_string) - 1}
+#define STR_NULL (str){NULL, 0}
+#define str_init(_string)  (str){_string, sizeof(_string) - 1}
 static inline void init_str(str *dest, const char *src)
 {
 	dest->s = (char *)src;