Reject numeric literals longer than DEC_MAX_DIGITS (999999999)

A signed-int overflow in decNumber's D2U macro lets huge literals
write attacker-controlled bytes past a stack buffer. Cap the length
before calling decNumberFromString, and pre-slice long strings in
jv_dump_string_trunc so the resulting error message doesn't itself
allocate a multi-GiB buffer.

Fixes CVE-2026-43894.

src/jv.c

@@ -570,7 +570,10 @@ static jvp_literal_number* jvp_literal_number_alloc(unsigned literal_length) {
 }
 
 static jv jvp_literal_number_new(const char * literal) {
-  jvp_literal_number* n = jvp_literal_number_alloc(strlen(literal));
+  size_t len = strlen(literal);
+  if (len > DEC_MAX_DIGITS)
+    return JV_INVALID;
+  jvp_literal_number* n = jvp_literal_number_alloc(len);
 
   decContext *ctx = DEC_CONTEXT();
   decContextClearStatus(ctx, DEC_Conversion_syntax);

src/jv_print.c

@@ -410,6 +410,10 @@ jv jv_dump_string(jv x, int flags) {
 
 char *jv_dump_string_trunc(jv x, char *outbuf, size_t bufsize) {
   assert(bufsize > 0);
+  if (jv_get_kind(x) == JV_KIND_STRING &&
+      (size_t)jv_string_length_bytes(jv_copy(x)) > bufsize) {
+    x = jv_string_slice(x, 0, bufsize);
+  }
   x = jv_dump_string(x, 0);
   const char *str = jv_string_value(x);
   const size_t len = strlen(str);