fixed a number of crashes due to empty string arguments or uninitialized registers

* An empty but valid teco_string_t can contain NULL pointers.
  More precisely, a state's done_cb() can be invoked with such empty strings
  in case of empty string arguments.
  Also a registers get_string() can return the NULL pointer
  for existing registers with uninitialized string parts.
* In all of these cases, the language should treat "uninitialized" strings
  exactly like empty strings.
* Not doing so, resulted in a number of vulnerabilities.
  * EN$$ crashed if "_" was uninitialized
  * The ^E@q and ^ENq string building constructs would crash for existing but
    uninitialized registers q.
  * ?$ would crash
  * ESSETILEXER$$ would crash
* This is now fixed.
  Test cases have been added.
* I cannot guarantee that I have found all such cases.
  Generally, it might be wise to change our definitions and make sure that
  every teco_string_t must have an associated heap object to be valid.
  All functions returning pointer+length pairs should consequently also never
  return NULL pointers.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/help.c b/src/help.c
index 48364de..9f589e1 100644
--- a/src/help.c
+++ b/src/help.c
@@ -283,10 +283,11 @@ teco_state_help_done(teco_machine_main_t *ctx, const teco_string_t *str, GError
 		                    "Help topic must not contain null-byte");
 		return NULL;
 	}
-	teco_help_topic_t *topic = teco_help_find(str->data);
+	const gchar *topic_name = str->data ? : "";
+	teco_help_topic_t *topic = teco_help_find(topic_name);
 	if (!topic) {
 		g_set_error(error, TECO_ERROR, TECO_ERROR_FAILED,
-		            "Topic \"%s\" not found", str->data);
+		            "Topic \"%s\" not found", topic_name);
 		return NULL;
 	}