gss_inquire_sec_context_by_oid(GSS_C_INQ_SSPI_SESSION_KEY) fails on
Windows because generic_gss_add_buffer_set_member() relies on the
ability to realloc() a null pointer. Unlike realloc(), HeapReAlloc()
requires an input pointer that (from the MSDN documentation) "is
returned by an earlier call to the HeapAlloc or HeapReAlloc function".
So gssalloc_realloc() must test for null inputs and call HeapAlloc()
Reported by Eric Pauly.
(cherry picked from commit d66b311093f1782c3610bbc77bd78fce411e8f79)