Report a missing 0xFF after a final BON8 string also without strict (#6119)

A BON8 string ends at the first byte that cannot continue it. When a
string ends a value and is not terminated by 0xFF, that byte belongs to
whatever follows the value, but the reader has already taken it from the
input and keeps it in its push-back buffer. With strict = true the
end-of-input check reports it; with strict = false the buffer was
discarded, so a following value in a stream lost its first byte or bytes
without an error ({0x61, 0x90} read "a" and then dropped the 0 that
follows).

Report parse_error.110 "string at the end of the value is not terminated
by 0xFF" when the push-back buffer is not empty once a BON8 value is
complete and strict is false, as bon8.md requires the final 0xFF. Output
of to_bon8 always has it, so canonical input is not affected.

Fixes #6110.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -18578,6 +18578,17 @@ class binary_reader
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
         }
 
+        // BON8: a string ends at the first byte that cannot continue it. If
+        // that byte follows the value, the reader has already taken it from
+        // the input and holds it in bon8_pushback. Without the end-of-input
+        // check below it would be dropped, and the next read would start
+        // behind it, so report the missing 0xFF instead.
+        if (result && !strict && input_format == input_format_t::bon8 && bon8_pushback_size != 0)
+        {
+            get_bon8();
+            return last_byte_error(exception_id::unexpected_end_of_input, "string at the end of the value is not terminated by 0xFF", "value", "");
+        }
+
         // strict mode: next byte must be EOF
         if (result && strict)
         {