Fix ECH error code: use BUFFER_ERROR for malformed peer input

Change innerClientHelloLen underflow guard in TLSX_ECH_Parse from
BAD_FUNC_ARG to BUFFER_ERROR to match the convention used throughout
tls.c for wire-protocol length/bounds validation.

src/tls.c

@@ -13606,7 +13606,7 @@ static int TLSX_ECH_Parse(WOLFSSL* ssl, const byte* readBuf, word16 size,
         /* read hello inner len */
         ato16(readBuf_p, &ech->innerClientHelloLen);
         if (ech->innerClientHelloLen < WC_AES_BLOCK_SIZE) {
-            return BAD_FUNC_ARG;
+            return BUFFER_ERROR;
         }
         ech->innerClientHelloLen -= WC_AES_BLOCK_SIZE;
         readBuf_p += 2;