unibi_from_term requires term to be non-NULL

Index: termkey.c
--- termkey.c.orig
+++ termkey.c
@@ -420,6 +420,9 @@ TermKey *termkey_new(int fd, int flags)
   termkey_set_flags(tk, flags);
 
   const char *term = getenv("TERM");
+  if (!term) {
+	  term = "xterm";
+  }
 
   if(!termkey_init(tk, term))
     goto abort;