Fixes incorrect indexing of KEX prefs string

After stripping out an invalid KEX pref entry, it would incorrectly advance again leaving invalid values in the list.

src/kex.c

@@ -3527,9 +3527,10 @@ libssh2_session_method_pref(LIBSSH2_SESSION * session, int method_type,
                 }
             }
         }
-
+        else {
             s = p ? (p + 1) : NULL;
         }
+    }
 
     if(strlen(newprefs) == 0) {
         LIBSSH2_FREE(session, newprefs);