Fix a bogus uninit variable warning

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2341)

ssl/statem/extensions.c

diff --git a/ssl/statem/extensions.c b/ssl/statem/extensions.c
index 99326b2..50fd3bb 100644
--- a/ssl/statem/extensions.c
+++ b/ssl/statem/extensions.c
@@ -1018,7 +1018,7 @@
                        != 0)) {
             const unsigned char *pcurves, *pcurvestmp, *clntcurves;
             size_t num_curves, clnt_num_curves, i;
-            unsigned int group_id;
+            unsigned int group_id = 0;
 
             /* Check if a shared group exists */