avoid LIMIT_HEAP integer multiplication wrap around (#184)

If a LIMIT_HEAP value once converted to bytes is larger than UINT_MAX
would result in a bogus setting that could trigger a matching failure
as shown by the following:

  PCRE2 version 10.42 2022-12-11
    re> /(*LIMIT_HEAP=4194304)a/
  data> a
  Failed: error -63: heap limit exceeded

Remove the multiplication and instead keep track of the maximum heap
allowed in KB as was done originally.

Aditionally, add a check to avoid overflowing a PCRE2_SIZE while
doubling the heap used and that could result in a crash (only on
systems with a 32-bit PCRE2_SIZE and using non standard settings).

Unlike the original, this code avoids rounding the heapframes_size
to the frame_size at the allocation time, which simplifies the logic
and wasn't really needed.

Fixes: d90fb238 (Refactor match_data() to always use the heap instead
       of having an initial frames vector on the stack..., 2022-07-27)
Closes: #183