Add a cast to silence an MSVC warning
Add a cast to silence an MSVC warning

This avoids this warning:
test/api/BaseThreadDecoderTest.cpp(280) : warning C4267: 'initializing' : conversion from 'size_t' to 'int32_t', possible loss of data

GuangweiWang/openh264,cisco/openh264,cisco/openh264,mstorsjo/openh264,GuangweiWang/openh264,mstorsjo/openh264,cisco/openh264,cisco/openh264,GuangweiWang/openh264,cisco/openh264,GuangweiWang/openh264,mstorsjo/openh264,GuangweiWang/openh264,cisco/openh264,GuangweiWang/openh264,cisco/openh264,mstorsjo/openh264,mstorsjo/openh264,GuangweiWang/openh264,mstorsjo/openh264,mstorsjo/openh264