Bug 722123 - Zero price entry added to price database on stock purchase

Ensure that a 0 price isn't written from the stock register.
Bug 721822 - GnuCash 2.6.0 loads data file much slower than 2.4.x

Profiling shows the main problem is checking the strings in the file for
valid XML and UTF-8. This does both in one pass, which cuts the time in
half, but it's still far slower than 2.4.x was.