Split diagnostics for two UTF-8 malformations

Split diagnostics for two UTF-8 malformations

Some UTF-8 sequences may have multiple malformations.  Commit
2b5e7bc2e60b4c4b5d87aa66e066363d9dce7930 tried to make sure that all
possible ones are raised, instead of abandoning searching after one is
found.  Since, I realized that there was yet another case of two
malformations that it returned only one or the other of.

An input buffer may be too short to fully express the code point it
purports to.  This can be determined by the first byte of the UTF-8
sequence indicating a longer sequence is requred than the space
available.  But also, that shortened sequence can have a premature
beginning of another character earlier than the shortness.  This commit
causes these to be both raised, instead of the previous behavior of
noting just one.