Fix read from uninitialized memory bug in GrpcBufferWriter.

This commit fixes an issue in which the following sequence of operations
leads to use of uninitialized memory:

1. Caller invokes GrpcBufferWriter::Next(), and then makes use of 8191
   bytes in the returned buffer (which is 8192 bytes in size).

2. Caller then returns the unused single byte via
   GrpcBufferWriter::BackUp(). This method invokes
   g_core_codegen_interface->grpc_slice_split_tail(), which causes
   backup_slice_ to be a grpc_slice with one byte.

3. At the next invocation of GrpcBufferWriter::Next(), a reference to
   the single byte grpc_slice is returned to the caller.

The problem here is that the returned reference is to the inlined buffer
in the grpc_slice, which is resident in slice_, not the location of the
buffer inside slice_buffer_ after
g_core_codegen_interface->grpc_slice_buffer_add() in
GrpcBufferWriter::Next(). As a result, any data the caller writes to the
returned void* data is lost.

The solution is to avoid inlined backup slices.