Add a bytestogroup macro

For converting the 2 byte group id into an unsigned int.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2341)

ssl/ssl_locl.h

@@ -2194,6 +2194,9 @@ SSL_COMP *ssl3_comp_find(STACK_OF(SSL_COMP) *sk, int n);
 # define TLS_CURVE_PRIME         0x0
 # define TLS_CURVE_CHAR2         0x1
 # define TLS_CURVE_CUSTOM        0x2
+
+#define bytestogroup(bytes) ((unsigned int)(bytes[0] << 8 | bytes[1]))
+
 __owur int tls1_ec_curve_id2nid(int curve_id, unsigned int *pflags);
 __owur int tls1_ec_nid2curve_id(int nid);
 __owur int tls1_check_curve(SSL *s, const unsigned char *p, size_t len);

ssl/statem/extensions.c

@@ -1039,7 +1039,7 @@ static int final_key_share(SSL *s, unsigned int context, int sent, int *al)
             /* Find the first group we allow that is also in client's list */
             for (i = 0, pcurvestmp = pcurves; i < num_curves;
                  i++, pcurvestmp += 2) {
-                group_id = pcurvestmp[0] << 8 | pcurvestmp[1];
+                group_id = bytestogroup(pcurvestmp);
 
                 if (check_in_list(s, group_id, clntcurves, clnt_num_curves, 1))
                     break;

ssl/statem/extensions_clnt.c

@@ -614,7 +614,7 @@ int tls_construct_ctos_key_share(SSL *s, WPACKET *pkt, unsigned int context,
             if (!tls_curve_allowed(s, pcurves, SSL_SECOP_CURVE_SUPPORTED))
                 continue;
 
-            curve_id = (pcurves[0] << 8) | pcurves[1];
+            curve_id = bytestogroup(pcurves);
             break;
         }
     }
@@ -1235,7 +1235,7 @@ int tls_parse_stoc_key_share(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
             return 0;
         }
         for (i = 0; i < num_curves; i++, pcurves += 2) {
-            if (group_id == (unsigned int)((pcurves[0] << 8) | pcurves[1]))
+            if (group_id == bytestogroup(pcurves))
                 break;
         }
         if (i >= num_curves