MINOR: dict: Store the length of the dictionary entries.

When allocating new dictionary entries we store the length of the strings.
May be useful so that not to have to call strlen() too much often at runing
time.
diff --git a/include/types/dict.h b/include/types/dict.h
index 9e4f41a..006e915 100644
--- a/include/types/dict.h
+++ b/include/types/dict.h
@@ -7,6 +7,7 @@
 struct dict_entry {
 	struct ebpt_node value;
 	unsigned int refcount;
+	size_t len;
 };
 
 struct dict {
diff --git a/src/dict.c b/src/dict.c
index 777530a..c2580e1 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -36,6 +36,7 @@
 	if (!de->value.key)
 		goto err;
 
+	de->len = strlen(s);
 	de->refcount = 1;
 
 	return de;
@@ -43,6 +44,7 @@
  err:
 	free(de->value.key);
 	de->value.key = NULL;
+	de->len = 0;
 	free(de);
 	return NULL;
 }