parser: silence 'no return value' warning
parser: silence 'no return value' warning

The code path should be unreachable so add an abort() statement. Fixes
the following warning:

    src/parser.c:1550: warning: control reaches end of non-void function

kevinhendricks/gumbo-parser,aleksandaratanasov/gumbo-parser,vmg/gumbo-parser,abc00/gumbo-parser,Sigil-Ebook/sigil-gumbo,luoxn28/gumbo-parser,abc00/gumbo-parser,f123h456/gumbo-parser,urandu/gumbo-parser,lvchao0428/gumbo-parser,f123h456/gumbo-parser,vmg/gumbo-parser,abc00/gumbo-parser,google/gumbo-parser,mcanthony/gumbo-parser,f123h456/gumbo-parser,Sigil-Ebook/sigil-gumbo,f123h456/gumbo-parser,luoxn28/gumbo-parser,aleksandaratanasov/gumbo-parser,aleksandaratanasov/gumbo-parser,kevinhendricks/gumbo-parser,lvchao0428/gumbo-parser,mcanthony/gumbo-parser,aleksandaratanasov/gumbo-parser,kevinhendricks/gumbo-parser,abc00/gumbo-parser,Sigil-Ebook/sigil-gumbo,luoxn28/gumbo-parser,aleksandaratanasov/gumbo-parser,urandu/gumbo-parser,f123h456/gumbo-parser,vmg/gumbo-parser,lvchao0428/gumbo-parser,abc00/gumbo-parser,google/gumbo-parser,lvchao0428/gumbo-parser,vmg/gumbo-parser,Sigil-Ebook/sigil-gumbo,urandu/gumbo-parser,google/gumbo-parser,mcanthony/gumbo-parser,google/gumbo-parser,luoxn28/gumbo-parser,luoxn28/gumbo-parser,kevinhendricks/gumbo-parser,urandu/gumbo-parser,Sigil-Ebook/sigil-gumbo,lvchao0428/gumbo-parser,kevinhendricks/gumbo-parser,google/gumbo-parser,urandu/gumbo-parser,mcanthony/gumbo-parser,mcanthony/gumbo-parser