Fix heap buffer overflow when formatting an empty string

The `jv_string_empty` did not properly null-terminate the string data,
which could lead to a heap buffer overflow. The test case of
GHSA-p7rr-28xf-3m5w (`0[""*0]`) was fixed by the commit dc849e9bb7,
but another case (`0[[]|implode]`) was still vulnerable. This commit
ensures string data is properly null-terminated, and fixes CVE-2025-48060.

src/jv.c

@@ -1148,6 +1148,7 @@ static jv jvp_string_empty_new(uint32_t length) {
   jvp_string* s = jvp_string_alloc(length);
   s->length_hashed = 0;
   memset(s->data, 0, length);
+  s->data[length] = 0;
   jv r = {JVP_FLAGS_STRING, 0, 0, 0, {&s->refcnt}};
   return r;
 }