Changelog entry for the mbedtls_mpi_read_xxx changes

mbedtls_mpi_read_binary{,_le} (in Mbed-TLS#4276)
and mbedtls_mpi_read_string (in Mbed-TLS#4644)
changed their behavior on an empty input from constructing an MPI object with
one limb to not allocating a limb. In principle, this change should be
transparent to applications, however it caused a bug in the library and it does
affect the value when writing back out, so list the change in the changelog.

Signed-off-by: Gilles Peskine <[email protected]>

ChangeLog.d/mpi_read_zero.txt

@@ -0,0 +1,9 @@
+Changes
+   * mbedtls_mpi_read_binary(), mbedtls_mpi_read_binary_le() and
+     mbedtls_mpi_read_string() now construct an mbedtls_mpi object with 0 limbs
+     when their input has length 0. Note that this is an implementation detail
+     and can change at any time, so this change should be transparent, but it
+     may result in mbedtls_mpi_write_binary() or mbedtls_mpi_write_string()
+     now writing an empty string where it previously wrote one or more
+     zero digits when operating from values constructed with an mpi_read
+     function and some mpi operations.