Speed up count_size_as_multibyte

Speed up count_size_as_multibyte

This function is used in many places to calculate the length of
a unibyte string converted to multibyte.

* src/character.c (count_size_as_multibyte): Move the overflow test
outside the loop, which makes it much faster.  Standard compilers
will even vectorise it if asked to (-O2 in Clang, -O3 in GCC).