Remove support for processing fragmented alerts

Prior to this change, BoringSSL maintained a 2-byte buffer for alerts,
and would support reassembly of fragmented alerts.

NSS does not support fragmented alerts, nor would any reasonable
implementation produce them. Remove fragmented alert handling and
produce an error if a fragmented alert has ever been encountered.

Change-Id: I31530ac372e8a90b47cf89404630c1c207cfb048
Reviewed-on: https://boringssl-review.googlesource.com/2125
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/ssl3.h b/include/openssl/ssl3.h
index 5083167..42e2154 100644
--- a/include/openssl/ssl3.h
+++ b/include/openssl/ssl3.h
@@ -372,10 +372,8 @@
 	SSL3_RECORD rrec;	/* each decoded record goes in here */
 	SSL3_RECORD wrec;	/* goes out from here */
 
-	/* storage for Alert/Handshake protocol data received but not
+	/* storage for Handshake protocol data received but not
 	 * yet processed by ssl3_read_bytes: */
-	unsigned char alert_fragment[2];
-	unsigned int alert_fragment_len;
 	unsigned char handshake_fragment[4];
 	unsigned int handshake_fragment_len;