Fixing two buffer overflow issues

[dutow]

Correct the size of tmp array

The snprintf using it should work with any int, which means this string
has to fit ",-2147483648\0", which is 12 characters + a null terminator.

Otherwise it could lead to a buffer overflow, corrupting other variables
on the stack, and maybe causing a crash too?

Fix resp_calls array size

This got broken during a refactoring in de71282. We need the plus 2
items because of the overflow/underflow bucket, without that we can
possibly overindex this array and corrupt other counters.

In earlier versions we even had the encoding variable stored directly
after this array, which possibly caused crashes or incorrectly working
string functions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.46%. Comparing base (71b045b) to head (9fd3141).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #592      +/-   ##
==========================================
- Coverage   85.48%   85.46%   -0.02%     
==========================================
  Files           3        3              
  Lines        1343     1342       -1     
  Branches      215      215              
==========================================
- Hits         1148     1147       -1     
  Misses         92       92              
  Partials      103      103              

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