Fix gssalloc_realloc() on Windows

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()
instead.

Reported by Eric Pauly.

(cherry picked from commit d66b311093f1782c3610bbc77bd78fce411e8f79)

https://github.com/krb5/krb5/commit/c1f08b6ff1b3988da72e64093e9818f15c83619f
Author: Greg Hudson <ghudson@mit.edu>
Commit: c1f08b6ff1b3988da72e64093e9818f15c83619f
Branch: krb5-1.17
 src/lib/gssapi/generic/gssapi_alloc.h |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)