Fix LIBSSH2_ALLOC checks.
Fix LIBSSH2_ALLOC checks.

These appear to be cut-and paste errors where the wrong variable is checked
for NULLness after calling LIBSSH2_ALLOC.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2