pack_sentence_for_*(): Fix crash w/ verbosity>=5 on short sentences
pack_sentence_for_*(): Fix crash w/ verbosity>=5 on short sentences

Repeat by:
echo x | link-parser -v=5

linas/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar