Bug 725054 - Re-numbering sub accounts crashes the program

g_strdup_printf ("%s-%0*d", prefix, num_digits, interval) calls malloc()
with the number of required characters, which includes num_digits characters
for the numeric. If the interval is 0, log10(num_children * interval) is
log10(0), which returns NAN (not a number), which when cast to size_t is
MAXINT. That generates a malloc error, which crashes GnuCash.

So don't let the interval be 0.
More fix-ups for the renumber subaccounts dialog.

Add more invariant checks for safety.
Suppress the '-' in the account-number format if the prefix is empty.
Set the value for the interval GtkSpinButton so that it won't be 0 on startup.
Fix the step-increment and page-increment property names in the builder file
so that they have an effect.