Fix unchecked i2d_PrivateKey() return value causing size_t overflow and abort

[metsw24-max]

The return value of i2d_PrivateKey() in ssl_asn1_table_set() is not validated before being assigned to an apr_size_t.

i2d_PrivateKey() returns an int and may return <= 0 on failure. This value is directly cast to apr_size_t, which can result in a large unsigned value when negative. This leads to excessive memory allocation attempts via ap_malloc()/ap_realloc(), causing the server to abort.

This patch introduces proper validation of the return value before casting, preventing invalid allocations.

Additionally, the caller in ssl_engine_pphrase.c is updated to handle NULL return values from ssl_asn1_table_set() to avoid NULL dereference.

Reproduction:

• Force i2d_PrivateKey() to return -1
• Apache attempts to allocate a huge buffer and aborts

After fix:

• Failure is handled gracefully
• No crash occurs