YJIT: Handle 0 total_exits YJIT Status Display

[dorkrawk]

Currently if there are 0 total_exits the YJIT stats display we see a lot of NaN's in the output due to dividing by 0 in the display code:

Before Example:

yjit_insns_count:            4955
ratio_in_yjit:              91.1%
avg_len_in_yjit:              5.0
Top-20 most frequent exit ops (NaN% of exits):
    trace_putobject_INT2FIX_1_:          0 (NaN%)
                           nop:          0 (NaN%)
                      getlocal:          0 (NaN%)
                      setlocal:          0 (NaN%)
                 getblockparam:          0 (NaN%)
                 setblockparam:          0 (NaN%)
...

This change gets rid of the "Top-20 most frequent exit ops..." line and replaces it with a total_exit count line

After Example:

yjit_insns_count:            3964
ratio_in_yjit:              89.1%
avg_len_in_yjit:              4.0
total_exits:                    0

[maximecb]

Well done Dave :)