[settings] Avoid overwriting the start of .text in fetch_string_setting()

fetch_string_setting() was subtracting one from the length of the
to-be-NUL-terminated buffer in order to obtain the length of the
unterminated buffer to be passed to fetch_setting().  This works
extremely well unless the length of the to-be-NUL-terminated buffer is
zero, at which point we end up giving fetch_setting() a buffer of
length -1UL, thereby inviting it to overwrite as much memory as it
wants...

src/core/settings.c

 int fetch_string_setting ( struct settings *settings, struct setting *setting,
 			   char *data, size_t len ) {
 	memset ( data, 0, len );
-	return fetch_setting ( settings, setting, data, ( len - 1 ) );
+	return fetch_setting ( settings, setting, data,
+			       ( ( len > 0 ) ? ( len - 1 ) : 0 ) );
 }
 
 /**