[readline] Silence spurious null pointer dereference Coverity warning

Coverity reported a potential null pointer dereference in readline.c,
which was fixed in commit b01781a ("[hci] Fix semantics of
replace_string() to match code comments").  With the fix in place,
Coverity still reports a potential null pointer dereference, even
though this cannot now happen.

Add an assertion to inform Coverity that the allocated string cannot
be NULL after a successful call to replace_string().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/readline.c

@@ -278,6 +278,7 @@ int readline_history ( const char *prompt, const char *prefill,
 	/* Prefill string */
 	if ( ( rc = replace_string ( &string, prefill ) ) != 0 )
 		goto error;
+	assert ( *line != NULL );
 	sync_console ( &string );
 
 	while ( 1 ) {