utf8.c: Don't use Newx in decoding UTF-8

utf8.c: Don't use Newx in decoding UTF-8

The bottom level UTF-8 decoding routine can be used during periods when
using Newx is prohibited, as diagnosed by Dave Mitchell for perl #130921
(see that ticket for his explanation).

This particular use of Newx was unnecessary, as it is just large enough
to hold a single character, and that can be done by an automatic
variable on the C stack.  The variable is used only upon rare error
conditions, but its only 14 bytes (15 on EBCDIC).