runtime: check for sysAlloc failures in pageAlloc

Change-Id: I78c5744bb01988f1f599569703d83fd21542ac7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/305911
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mpagealloc.go

@@ -395,6 +395,9 @@ func (p *pageAlloc) grow(base, size uintptr) {
 			// Store it atomically to avoid races with readers which
 			// don't acquire the heap lock.
 			r := sysAlloc(unsafe.Sizeof(*p.chunks[0]), p.sysStat)
+			if r == nil {
+				throw("pageAlloc: out of memory")
+			}
 			atomic.StorepNoWB(unsafe.Pointer(&p.chunks[c.l1()]), r)
 		}
 		p.chunkOf(c).scavenged.setRange(0, pallocChunkPages)