Fix bug in mono-buffer.fun introduced by 8d5681c#36

MatthewFluet wants to merge 1 commit intoSMLFamily:masterSMLFamily/BasisLibrary:masterfrom

MatthewFluet:mono-buffer-ensureCapcity-bugfixMatthewFluet/BasisLibrary:mono-buffer-ensureCapcity-bugfixCopy head branch name to clipboard