runtime: fix buffer overflow in stringtoslicerune

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