libssh2: Fix of 2 CVEs

CVE-2019-13115: add bounds-checked stringbuf helpers and use them in diffiehellmansha1 to prevent out-of-bounds read on malformed KEX reply - CVE-2019-17498: harden bounds checks in SSHMSGDISCONNECT, SSHMSGDEBUG and SSHMSGGLOBALREQUEST handlers to prevent integer overflow / out-of-bounds read...