[PATCH] knownhost_check(): Don't dereference ext if NULL is passed

Documentation for libssh2_knownhost_checkp() and related functions
states that the last argument is filled with data if non-NULL.

"knownhost if set to non-NULL, it must be a pointer to a 'struct
libssh2_knownhost' pointer that gets filled in to point to info about a
known host that matches or partially matches."

In this function ext is dereferenced even if set to NULL, causing
segfault in applications not needing the extra data.

---
 src/knownhost.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/src/knownhost.c b/src/knownhost.c
index d90f1d4..193bda3 100644
--- a/src/knownhost.c
+++ b/src/knownhost.c
@@ -417,7 +417,8 @@ knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
                 /* host name match, now compare the keys */
                 if(!strcmp(key, node->key)) {
                     /* they match! */
-                    *ext = knownhost_to_external(node);
+                    if (ext)
+                        *ext = knownhost_to_external(node);
                     badkey = NULL;
                     rc = LIBSSH2_KNOWNHOST_CHECK_MATCH;
                     break;
@@ -438,7 +439,8 @@ knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
     if(badkey) {
         /* key mismatch */
-        *ext = knownhost_to_external(badkey);
+        if (ext)
+            *ext = knownhost_to_external(badkey);
         rc = LIBSSH2_KNOWNHOST_CHECK_MISMATCH;
     }
-- 
1.7.3.4
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