Note that EVP_CIPHER_get_iv_length returns negative values on error

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/18875)

doc/man3/EVP_EncryptInit.pod

@@ -1234,7 +1234,7 @@ length.
 EVP_CIPHER_CTX_set_padding() always returns 1.
 
 EVP_CIPHER_get_iv_length() and EVP_CIPHER_CTX_get_iv_length() return the IV
-length or zero if the cipher does not use an IV.
+length, zero if the cipher does not use an IV and a negative value on error.
 
 EVP_CIPHER_CTX_get_tag_length() return the tag length or zero if the cipher
 does not use a tag.