Add sanity check in ssl3_cbc_digest_record

For SSLv3 the code assumes that |header_length| > |md_block_size|. Whilst
this is true for all SSLv3 ciphersuites, this fact is far from obvious by
looking at the code. If this were not the case then an integer overflow
would occur, leading to a subsequent buffer overflow. Therefore I have
added an explicit sanity check to ensure header_length is always valid.
Thanks to Kevin Wojtysiak (Int3 Solutions) and Paramjot Oberoi (Int3
Solutions) for reporting this issue.
Reviewed-by: NAndy Polyakov <appro@openssl.org>

ssl/s3_cbc.c

@@ -397,12 +397,22 @@ void ssl3_cbc_digest_record(const EVP_MD_CTX *ctx,
 
     if (k > 0) {
         if (is_sslv3) {
+            unsigned overhang;
+
             /*
              * The SSLv3 header is larger than a single block. overhang is
              * the number of bytes beyond a single block that the header
-             * consumes: either 7 bytes (SHA1) or 11 bytes (MD5).
+             * consumes: either 7 bytes (SHA1) or 11 bytes (MD5). There are no
+             * ciphersuites in SSLv3 that are not SHA1 or MD5 based and
+             * therefore we can be confident that the header_length will be
+             * greater than |md_block_size|. However we add a sanity check just
+             * in case
              */
-            unsigned overhang = header_length - md_block_size;
+            if (header_length <= md_block_size) {
+                /* Should never happen */
+                return;
+            }
+            overhang = header_length - md_block_size;
             md_transform(md_state.c, header);
             memcpy(first_block, header + md_block_size, overhang);
             memcpy(first_block + overhang, data, md_block_size - overhang);