wstream: Change wstream_write failure behavior

Before this change, any write that could cause a WStream instance to use more
than `maxmem` would fail, which is not acceptable when writing big chunks of
data. (This could happen when returning contents from a big buffer through the
API, for example).

Writes of any size are now allowed, but before we check if the currently used
memory doesn't break the limit. This should be enough to prevent us from
stacking data when talking to a locked process.

src/nvim/os/wstream.c

@@ -90,7 +90,7 @@ bool wstream_write(WStream *wstream, WBuffer *buffer)
   // This should not be called after a wstream was freed
   assert(!wstream->freed);
 
-  if (wstream->curmem + buffer->size > wstream->maxmem) {
+  if (wstream->curmem > wstream->maxmem) {
     return false;
   }