stream: stop randomly corrupting memory

The intent of the line above the modified one code was raising the
amount of read data, so that many stream_peek() calls with small len
values would not degrade performance by effectively turning every
stream_peak() into an unbuffered read call to the stream implementation.
So this confusing looking MPMAX() was correct, but "chunk" could still
get beyond the buffer.

So just fix that and limit "chunk" correctly.

I'm not sure whether the commit referenced below accidentally removed
some intricate guarantee that this couldn't happen, since the code was
around since 2013. It could have relied on TOTAL_BUFFER_SIZE >
STREAM_BUFFER_SIZE. But not sure. I've rewritten all this code in my own
branch a year ago, so who knows.

Fixes: 162e0f5ad9
Fixes: #6948

stream/stream.c

@@ -402,7 +402,8 @@ struct bstr stream_peek(stream_t *s, int len)
         // Fill rest of the buffer.
         while (buf_valid < len) {
             int chunk = MPMAX(len - buf_valid, STREAM_BUFFER_SIZE);
-            assert(buf_valid + chunk <= TOTAL_BUFFER_SIZE);
+            if (buf_valid + chunk > TOTAL_BUFFER_SIZE)
+                chunk = TOTAL_BUFFER_SIZE - buf_valid;
             int read = stream_read_unbuffered(s, &s->buffer[buf_valid], chunk);
             if (read == 0)
                 break; // EOF