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Your problem is reminiscent of the Osgood's criterion, which can be found on MO herehere. Local integrability near $0$ for $b$ is only what you need.
A proof of this criterion may be found for instance here, page 55.
Your problem is reminiscent of the Osgood's criterion, which can be found on MO here. Local integrability near $0$ for $b$ is only what you need.