af-alg: Fix output offset if not all data was processed during en-/decryption

If only parts of the total data could be written to the kernel, the result
of the next read chunk would incorrectly get written at the beginning of
the output buffer again.

Also makes sure to close the accepted FD in error cases.

Fixes: 1b5de7ce3b ("Use a generic AF_ALG wrapper for common operations")

src/libstrongswan/plugins/af_alg/af_alg_ops.c

@@ -167,6 +167,7 @@ METHOD(af_alg_ops_t, crypt_, bool,
 				continue;
 			}
 			DBG1(DBG_LIB, "writing to AF_ALG crypter failed: %s", strerror(errno));
+			close(op);
 			return FALSE;
 		}
 		while (read(op, out, len) != len)
@@ -175,10 +176,12 @@ METHOD(af_alg_ops_t, crypt_, bool,
 			{
 				DBG1(DBG_LIB, "reading from AF_ALG crypter failed: %s",
 					 strerror(errno));
+				close(op);
 				return FALSE;
 			}
 		}
 		data = chunk_skip(data, len);
+		out += len;
 		/* no IV for subsequent data chunks */
 		msg.msg_controllen = 0;
 	}