DTLS export: cap IV size at buffer size

ExportKeyState was writing ssl->specs.iv_size bytes from
keys->aead_enc_imp_IV (always sized AEAD_MAX_IMP_SZ). ssl->specs.iv_size
carries a different meaning depending on the cipher suite: in AEAD
suites it's the implicit IV / nonce size, but in CBC it's the block
cipher's IV size (16). In CBC this overran the size of aead_enc_imp_IV
(12).

Author: mattia-moffa

src/internal.c

@@ -1000,7 +1000,9 @@ static int ExportKeyState(WOLFSSL* ssl, byte* exp, word32 len, byte ver,
     XMEMCPY(exp + idx, keys->aead_exp_IV, AEAD_MAX_EXP_SZ);
     idx += AEAD_MAX_EXP_SZ;
 
-    sz = (small)? 0: ssl->specs.iv_size;
+    sz = (small) ? 0 :
+         (ssl->specs.iv_size > AEAD_MAX_IMP_SZ ? AEAD_MAX_IMP_SZ
+                                               : ssl->specs.iv_size);
     if (idx + (sz * 2) + OPAQUE8_LEN > len) {
         WOLFSSL_MSG("Buffer not large enough for imp IVs");
         return BUFFER_E;