stransport: do not use `git_stream_free` on uninitialized stransport

When failing to initialize a new stransport stream, we try to
release already allocated memory by calling out to
`git_stream_free`, which in turn called out to the stream's
`free` function pointer. As we only initialize the function
pointer later on, this leads to a `NULL` pointer exception.

Furthermore, plug another memory leak when failing to create the
SSL context.

src/stransport_stream.c

@@ -259,6 +259,7 @@ int git_stransport_stream_new(git_stream **out, const char *host, const char *po
 	st->ctx = SSLCreateContext(NULL, kSSLClientSide, kSSLStreamType);
 	if (!st->ctx) {
 		giterr_set(GITERR_NET, "failed to create SSL context");
+		git__free(st);
 		return -1;
 	}
 
@@ -268,7 +269,8 @@ int git_stransport_stream_new(git_stream **out, const char *host, const char *po
 	    (ret = SSLSetProtocolVersionMin(st->ctx, kTLSProtocol1)) != noErr ||
 	    (ret = SSLSetProtocolVersionMax(st->ctx, kTLSProtocol12)) != noErr ||
 	    (ret = SSLSetPeerDomainName(st->ctx, host, strlen(host))) != noErr) {
-		git_stream_free((git_stream *)st);
+		CFRelease(st->ctx);
+		git__free(st);
 		return stransport_error(ret);
 	}