Fix to_msgpack() reading the inactive number union member (#5694)

* Fix to_msgpack() reading the inactive number union member

basic_json stores number_integer and number_unsigned in a union, and
number_unsigned_t only has to be at least as wide as number_integer_t
(with the default types, both are 64-bit and have the same
representation). When number_integer_t is narrower, write_msgpack()
read the wrong union member in two places:

- The number_unsigned case wrote number_integer's bits instead of
  number_unsigned's, silently writing the wrong value whenever it
  did not fit in number_integer_t.
- The number_integer case (non-negative branch) picked the encoded
  width by comparing number_unsigned's bits, which is undefined
  behavior, though the value written was still number_integer's, so
  at worst a too-wide encoding was chosen.

Read the active member in both cases, like the other binary writers
(CBOR, UBJSON, BJData, BSON, BON8) already do.

Fixes #5644.

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

* Cast number_integer to number_unsigned_t only once in to_msgpack()

Addresses review comment by @gregmarr.

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

---------

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

docs/mkdocs/docs/api/basic_json/to_msgpack.md

@@ -76,3 +76,6 @@ Linear in the size of the JSON value `j`.
 
 - Added in version 2.0.9.
 - Throws `out_of_range.412` and `out_of_range.415` since version 3.13.0.
+- Fixed in version 3.13.0 to serialize `number_integer_t`/`number_unsigned_t` pairs of different width correctly;
+  before, integers could be serialized with the wrong value if `number_integer_t` was narrower than
+  `number_unsigned_t`.