runtime: fix buffer overflow in stringtoslicerune

On 32-bits n*sizeof(r[0]) can overflow.
Or it can become 1<<32-eps, and mallocgc will "successfully"
allocate 0 pages for it, there are no checks downstream
and MHeap_Grow just does:
npage = (npage+15)&~15;
ask = npage<<PageShift;

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews
https://golang.org/cl/54760045

src/pkg/runtime/malloc.goc

@@ -224,6 +224,8 @@ largealloc(uint32 flag, uintptr *sizep)
 
 	// Allocate directly from heap.
 	size = *sizep;
+	if(size + PageSize < size)
+		runtime·throw("out of memory");
 	npages = size >> PageShift;
 	if((size & PageMask) != 0)
 		npages++;

src/pkg/runtime/string.goc

@@ -334,6 +334,8 @@ func stringtoslicerune(s String) (b Slice) {
 		n++;
 	}
 
+	if(n > MaxMem/sizeof(r[0]))
+		runtime·throw("out of memory");
 	mem = runtime·roundupsize(n*sizeof(r[0]));
 	b.array = runtime·mallocgc(mem, 0, FlagNoScan|FlagNoZero);
 	b.len = n;