Return an error if the buffer is too large.

So far, we would case a uint64 to a int32 but that may result in an
integer overflow. At this point, GoBytes does not seem to support
buffers this large, so the next best thing to do is return an error if
the given buffer is too large.

buffer.go

@@ -8,6 +8,7 @@ import "C"
 import (
 	"bytes"
 	"fmt"
+	"math"
 	"runtime"
 	"unsafe"
 )
@@ -148,6 +149,10 @@ func (b *Buffer) dataCopy() ([]byte, error) {
 		// Since this buffer is TileDB-managed, make sure it's not GC'd before we're
 		// done with its memory.
 		defer runtime.KeepAlive(b)
+
+		if csize > math.MaxInt32 {
+			return nil, fmt.Errorf("TileDB's buffer (%d) larger than maximum allowed CGo buffer (%d)", csize, math.MaxInt32)
+		}
 		return C.GoBytes(cbuffer, C.int(csize)), nil
 	}