Remove Extraneous bytes from buffer post pem write

In order to remove large buffers from the stack, the der data is written
into the same buffer that the pem is eventually written into, however
although the pem data is zero terminated, there is now data left in the
buffer after the zero termination, which can cause
mbedtls_x509_crt_parse to fail to parse the same buffer if passed back
in. Patches also applied to mbedtls_pk_write_pubkey_pem, and
mbedtls_pk_write_key_pem, which use similar methods of writing der data
to the same buffer, and tests modified to hopefully catch any future
regression on this.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

ChangeLog.d/clean_pem_buffers.txt

@@ -0,0 +1,6 @@
+Bugfix
+  * In PEM writing functions, fill the trailing part of the buffer with null
+    bytes. This guarantees that the corresponding parsing function can read
+    the buffer back, which was the case for mbedtls_x509write_{crt,csr}_pem
+    until this property was inadvertently broken in Mbed TLS 2.19.0.
+    Fixes #3682.