regedit: Prevent out-of-bounds reads when unescaping a string (Valgrind).

programs/regedit/regproc.c

@@ -403,6 +403,7 @@ static BOOL REGPROC_unescape_string(WCHAR *str, WCHAR **unparsed)
                 str[val_idx] = str[str_idx];
                 break;
             default:
+                if (!str[str_idx]) return FALSE;
                 output_message(STRING_ESCAPE_SEQUENCE, str[str_idx]);
                 str[val_idx] = str[str_idx];
                 break;