Allow ATHandler::read_int to return 0 successfully

[dextero]

Description

Currently, ATHandler::read_int is documented to return a positive integer on success, -1 on error. In cases where std::strtol fails to parse a value, 0 is returned - and it is impossible to determine whether that 0 means "parsing failed" or "parsed 0 successfully".

Most places in mbed-os that actually do error checking depend on 0 meaning success, e.g.:

• mbed-os/features/cellular/framework/targets/QUECTEL/BG96/QUECTEL_BG96_CellularStack.cpp

  Line 114 in 818c1d6

  if (_at.read_int() == 0) {

• mbed-os/features/cellular/framework/targets/QUECTEL/BG96/QUECTEL_BG96_CellularContext.cpp

  Line 137 in 818c1d6

  int err = _at.read_int();

• mbed-os/features/cellular/framework/targets/UBLOX/N2XX/UBLOX_N2XX_CellularStack.cpp

  Line 96 in 60d047c

  if ((_at.get_last_error() != NSAPI_ERROR_OK) || (sock_id == -1)) {

• mbed-os/features/cellular/framework/targets/UBLOX/AT/UBLOX_AT_CellularContext.cpp

  Line 361 in 8767ab4

  case 0:

This pull request updates ATHandler::read_int so that it only returns 0 on success, and updates the method comment appropriately.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[ciarmcom]

@dextero, thank you for your changes.
@ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[bulislaw]

@AnttiKauppila please review

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

This PR is part of rollup PR #11366

Please do not update this PR, or if needed, let us know asap