Fixed tls_prf() to handle keys with odd length

The middle byte of the secret (key for PRF) is shared with key halfs in
case the key length is odd. This does not happen in any of the current
tls_prf() uses, but it's better to fix this function to avoid future issues
should someone end up defining a use that uses an odd length for the key.

src/crypto/sha1.c

@@ -265,6 +265,10 @@ int tls_prf(const u8 *secret, size_t secret_len, const char *label,
 	L_S1 = L_S2 = (secret_len + 1) / 2;
 	S1 = secret;
 	S2 = secret + L_S1;
+	if (secret_len & 1) {
+		/* The last byte of S1 will be shared with S2 */
+		S2--;
+	}
 
 	hmac_md5_vector(S1, L_S1, 2, &MD5_addr[1], &MD5_len[1], A_MD5);
 	hmac_sha1_vector(S2, L_S2, 2, &SHA1_addr[1], &SHA1_len[1], A_SHA1);