Handle cut-off UTF-8 sequences when reading files

Read additional bytes from the file to complete the UTF-8 sequence so the bytes
in it don't get converted to U+FFFD replacement characters.

tests/jq.test

@@ -1312,4 +1312,3 @@ jq: error: syntax error, unexpected INVALID_CHARACTER, expecting $end (Unix shel
 (.[{}] = 0)?
 null
 
-