ping: fix buffer allocator error handling

The code sets f->n_vectors = n_to_send, but it can bail out of the
loop if vlib_buffer_copy(...) returns 0.

Need to fix f->n_vectors in the error return path, or we enqueue some
number of 0xfefefefe buffer indices in a debug image or worse in a
production image.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I2d886266006c6c1c2f9ef8e3b95eb46ac6c0b3df