Fix signed integer overflow in exponent parsing

[grisumbras]

Fix #612

[codecov]

Codecov Report

Merging #614 (5b45854) into develop (3f48a27) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #614   +/-   ##
========================================
  Coverage    99.05%   99.05%           
========================================
  Files           68       68           
  Lines         6114     6119    +5     
========================================
+ Hits          6056     6061    +5     
  Misses          58       58           

Impacted Files	Coverage Δ
include/boost/json/basic_parser_impl.hpp	99.19% <100.00%> (+<0.01%)	⬆️

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[pauldreik]

I can (in addition to the fuzz CI job with the test cases) confirm this fixes the problem the fuzzer found. I tested all the test cases from https://oss-fuzz.com/testcases?q=group%3A5898123298340864

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/614/pullrequest.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/614/pullrequest.html