Keep a NUL byte ending a // comment as the end of input

With the default NUL handling (JSON_STRICT_NUL_HANDLING not set), a NUL
byte in the input is treated as the real end of input everywhere -
except when it immediately ends a `//` comment: scan_comment() matched
'\0' as a comment terminator like '\n', so the NUL was consumed as
part of the comment and scan() never saw it as end of input; the next
get() then kept reading past it. Multi-line comments and
JSON_STRICT_NUL_HANDLING=1 were unaffected, since there the NUL is
just part of the comment text.

Fix scan_comment() to leave the NUL unconsumed (unget()) instead of
returning it as part of the comment, so the following scan() reports
it as end of input, exactly as for a NUL anywhere else.

Fixes #5659.

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

include/nlohmann/detail/input/lexer.hpp

@@ -975,10 +975,15 @@ class lexer : public lexer_base<BasicJsonType>
                         case '\n':
                         case '\r':
                         case char_traits<char_type>::eof():
+                            return true;
+
 #if !JSON_STRICT_NUL_HANDLING
                         case '\0':
-#endif
+                            // a NUL byte is the end of the input (see scan()),
+                            // so leave it for scan() to see
+                            unget();
                             return true;
+#endif
 
                         default:
                             break;