(strpoolcollect): allow p == NULL, return the empty string (allocated)

lib/roken/strpool.c

@@ -100,7 +100,10 @@ rk_strpoolprintf(struct rk_strpool *p, const char *fmt, ...)
 char * ROKEN_LIB_FUNCTION
 rk_strpoolcollect(struct rk_strpool *p)
 {
-    char *str = p->str;
+    char *str;
+    if (p == NULL)
+	return strdup("");
+    str =  = p->str;
     p->str = NULL;
     free(p);
     return str;