#4689 Empty string at spanner bound causes infinite loop

[Simon Albrecht]

The following snippet:

 \version "2.19.25"
{
  1-\tweak bound-details.left.text "" \startTrillSpan
  1\stopTrillSpan
}

yields apparently infinite occurences of

programming error: Improbable offset for stencil: 1000001.000000 staff space
Setting to zero.

and I have to abort the program.

With 2.19.22, the same code gives

programming error: Improbable offset for stencil: -inf staff space
programming error: Improbable offset for stencil: -nan staff space
warning: Found infinity or nan in output. Substituting 0.0

and no sensible output.