Fix -Wmaybe-uninitialized in test/c/std/json
diff --git a/test/c/std/json.c b/test/c/std/json.c
index a369caa..0819511 100644
--- a/test/c/std/json.c
+++ b/test/c/std/json.c
@@ -1307,7 +1307,7 @@
       if ((vbc == WUFFS_BASE__TOKEN__VBC__STRING) &&
           (vbd ==
            WUFFS_BASE__TOKEN__VBD__STRING__CONVERT_1_DST_4_SRC_BACKSLASH_X)) {
-        uint8_t b[8];
+        uint8_t b[8] = {0};
         size_t n = wuffs_base__hexadecimal__decode4(
             wuffs_base__make_slice_u8(&b[0], 8),
             wuffs_base__make_slice_u8(src_slice.ptr + src_index, token_length));