box: validate key_def->part_count prior to memory allocation

part_count was checked in index_def_check(), which was called too late.
Before that check:
1. `malloc(sizeof(*part_def) * part_count)` can fail for huge part_count;
2. key_def_new() can crash for zero part_count because of out of bound
   access in:

NO_WRAP
   - #1 key_def_contains_sequential_parts (def=0x5555561a2ef0) at src/box/tuple_extract_key.cc:26
   - #2 key_def_set_extract_func (key_def=0x5555561a2ef0) at src/box/tuple_extract_key.cc:442
   - #3 key_def_set_func (def=0x5555561a2ef0) at src/box/key_def.c:162
   - #4 key_def_new (parts=0x7fffc4001350, part_count=0, for_func_index=false) at src/box/key_def.c:320
NO_WRAP

Closes #8688

NO_DOC=bugfix

(cherry picked from commit ef9e3320)