Cygwin: cygheap: fix fork error after heap has grown

Cygwin: cygheap: fix fork error after heap has grown

2f9b8ff0 introduced a problem where forks would sometimes fail with:

child_copy: cygheap read copy failed, 0x0..0x80044C750, done 0, windows pid 14032, Win32 error 299

When cygheap_max was > CYGHEAP_STORAGE_INITIAL, commit_size would be set to
allocsize(cygheap_max), which is an address, not a size.  VirtualAlloc would be
called to commit commit_size bytes, which would fail, and then child_copy would
be called with zero as the base address.

Fixes: 2f9b8ff00cce ("Cygwin: decouple cygheap from Cygwin DLL")
Signed-off-by: David McFarland <corngood@gmail.com>