virBufferAdd: Ensure that the buffer is initialized also when len == 0

virBufferAdd: Ensure that the buffer is initialized also when len == 0

There's an optimization in virBufferAdd which returns early when the
length of the added string is 0 (given that auto-indent is disabled).

The optimization causes inconsistent behaviour between these two cases:

 virBufferAdd(buf, "", 0);  // this doesn't initialize the buffer

and

 virBufferAdd(buf, "", -1); //this initializes the buffer

Since using an empty string is used to prime the buffer to an empty
string it can be confusing. Remove the optimization.

This fixes such a wrong initialization done in x86FeatureNames.

Note that our code in many places expects that if no virBuffer APIs are
used on a buffer object, then NULL should be retured, so we can't always
prime the buffer to an empty string.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>