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, 1 insertions, 0 deletions

diff --git a/src/core-commands.c b/src/core-commands.c
index bceb644..5f6ec53 100644
--- a/src/core-commands.c
+++ b/src/core-commands.c
@@ -1418,6 +1418,7 @@ teco_state_changedir_done(teco_machine_main_t *ctx, const teco_string_t *str, GE
 			                    "Null-character not allowed in filenames");
 			return NULL;
 		}
+		g_assert(home.data != NULL);
 
 		g_free(dir);
 		dir = home.data;