Fix get_conversion_field --with-wide-wint overflow

Fix get_conversion_field --with-wide-wint overflow

* src/textconv.c (get_conversion_field): Set max value to
PTRDIFF_MAX, not MOST_POSITIVE_FIXNUM, since the variable is
ptrdiff_t, not EMACS_INT.  Problem caught by gcc -Woverflow on a
32-bit platform with --with-wide-int.

(cherry picked from commit b4050ab75e896dd0df51624c956e0dd412dde2cc)