bidi_test: minor memory fixes.

Spotted by Coverity: if you _just_ gave a filename to bidi_test,
without any previous argument that set testfn to something other than
NULL, the program would crash rather than giving an error message.

(It's only a test program, but test programs you only run once in a
blue moon are the ones that _most_ need to explain their command-line
syntax to you carefully, because you've forgotten it since last time
you used them!)

Also, conditionalised a memcpy on the size not being 0, because it's
illegal to pass a null pointer to memcpy _even_ if size==0. (That
would only happen with a test case containing a zero-length string,
but whatever.)

terminal/bidi_test.c

@@ -44,6 +44,7 @@ static void run_test(const char *filename, unsigned lineno,
 {
     size_t bcs_orig_len = bcs_len;
     bidi_char *bcs_orig = snewn(bcs_orig_len, bidi_char);
+    if (bcs_orig_len)
         memcpy(bcs_orig, bcs, bcs_orig_len * sizeof(bidi_char));
 
     bcs_len = do_bidi_test(ctx, bcs, bcs_len, override);
@@ -335,6 +336,12 @@ int main(int argc, char **argv)
         } else {
             const char *filename = arg;
 
+            if (!testfn) {
+                fprintf(stderr, "no mode argument provided before filename "
+                        "'%s'\n", filename);
+                return 1;
+            }
+
             if (!strcmp(filename, "-")) {
                 testfn("<standard input>", stdin);
             } else {