clang-tidy warns about operations on uninitialized values

[farmaazon]

dtoa.h:233:3: warning: The expression is an uninitialized value. The computed value will also be garbage [clang-analyzer-core.uninitialized.Assign] buffer[len - 1]--;

As far as I could reach out, there is only one situation, when there is unsafe operation in this line; namely, when len == 0. But I can't deduce if this condition is always met in this place.

Here is more output from clang-tidy:
dtoa_warning.txt