ds: string: set iterator value to B_WCHAR_INVALID when end of string is reached

ds/string.c

@@ -8,7 +8,8 @@
 #include <stdlib.h>
 #include <string.h>
 
-/* maximum length of string that can be stored inline, not including null-terminator */
+/* maximum length of string that can be stored inline, not including
+ * null-terminator */
 #define STRING_INLINE_CAPACITY 15
 
 #define IS_VALID_UTF8_SCALAR(x)                                                \
@@ -243,7 +244,8 @@ static int string_change_capacity(struct b_string_p *str, size_t capacity)
 	bool is_now_inline = capacity == STRING_INLINE_CAPACITY;
 
 	if (capacity == old_capacity) {
-		/* this also handles the case where the old and new capacity both fit into the inline buffer. */
+		/* this also handles the case where the old and new capacity
+		 * both fit into the inline buffer. */
 		return 0;
 	}
 
@@ -1447,6 +1449,7 @@ static enum b_status chars_iterator_move_next(struct b_string_iterator_p *it)
 	it->codepoint_index += 1;
 
 	if (it->byte_index >= it->_s_p->s_len) {
+		it->char_value = B_WCHAR_INVALID;
 		return B_ERR_NO_DATA;
 	}