A sequence of <digits>e<mbhead><mbtail>, e.g.:

  SELECT 123eXYzzz FROM t1;

was not scanned correctly (where XY is a multi-byte character).

The multi-byte head byte X was appended to 123e separately from
the multi-byte tail byte Y, so a pointer to "Yzzz" was passed
into scan_ident_start(), which failed on a bad multi-byte sequence.

After this change, scan_ident_start() gets a pointer to "XYzzz",
so it correctly sees the whole multi-byte character.