openssl: fix compile-time warning in Curl_ossl_check_cxn()

... introduced in curl-7_48_0-293-g2968c839:

Error: COMPILER_WARNING:
lib/vtls/openssl.c: scope_hint: In function ‘Curl_ossl_check_cxn’
lib/vtls/openssl.c:767:15: warning: conversion to ‘int’ from ‘ssize_t’
may alter its value [-Wconversion]