Fix crash when a stencil routine is missing

[hanwenn]

Can this be made so that the regtests and similar crash when an unknown
stencil expression is executed?  Perhaps a -d option could help here.


http://codereview.appspot.com/83046/diff/1/5
File scm/output-svg.scm (right):

http://codereview.appspot.com/83046/diff/1/5#newcode284
Line 284: (if (< 0 (string-length expr))
isnt there a function string-empty? or string-null? (or similar) to do
this in a more roundabout name?

http://codereview.appspot.com/83046