Fix functions that rely on nonportable malloc behaviour

[kirill-shevtsov]

From man malloc:

Nonportable behavior
The behavior of these functions when the requested size is zero is glibc specific; other implementations may return NULL without setting errno, and portable POSIX programs should tolerate such behavior.

Two unit tests with empty plain text will fail on systems where malloc(0) returns NULL, e.g., AIX:

check_jwe.c:172:F:core:test_cjose_jwe_self_encrypt_self_decrypt_empty:0: cjose_jwe_encrypt [RSA-OAEP/A128GCM] failed: out of memory, file: src/jwe.c, function: _cjose_jwe_malloc, line: 198
check_jws.c:113:F:core:test_cjose_jws_self_sign_self_verify_empty:0: cjose_jws_sign [PS256] failed: out of memory, file: src/jws.c, function: _cjose_jws_build_dat, line: 145

Also test_cjose_jwe_self_encrypt_self_decrypt_many will fail for RSA-OAEP + A128GCM if the first random byte it generates is zero.