fix fmemopen write-mode streams clobbering final byte with null

commit d2e061a2bd3f7674cfef2e2217e0695419041b5e wrongly added this
case based on a misreading of the standard text regarding null
termination in write/update modes. apparently, the condition "if it
fits" was interpreted as only applying to update modes, despite the
words "or for writing only" appearing.