runtime: handle sysReserve failure in mHeap_SysAlloc

sysReserve will return nil on failure - correctly handle this case and return
nil to the caller. Currently, a failure will result in h.arena_end being set
to psize, h.arena_used being set to zero and fun times ensue.

On the openbsd/arm builder this has resulted in:

  runtime: address space conflict: map(0x0) = 0x40946000
  fatal error: runtime: address space conflict

When it should be reporting out of memory instead.

Change-Id: Iba828d5ee48ee1946de75eba409e0cfb04f089d4
Reviewed-on: https://go-review.googlesource.com/15056Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc.go

@@ -398,6 +398,9 @@ func mHeap_SysAlloc(h *mheap, n uintptr) unsafe.Pointer {
 			// is reserved and part is not.
 			var reserved bool
 			p := uintptr(sysReserve((unsafe.Pointer)(h.arena_end), p_size, &reserved))
+			if p == 0 {
+				return nil
+			}
 			if p == h.arena_end {
 				h.arena_end = new_end
 				h.arena_reserved = reserved