rb tree: change INSERT_COLOR to avoid always-true test

[dougm]

Summary

Separate the first iteration of the loop in RB_INSERT_COLOR from the rest. That iteration always reaches the 'continue' statement, and gcc compilers, not knowing that, issue warnings about the use of an uninitialized 'child' variable. By separating the first iteration, the always-true test is not made, saving one instruction, but at the cost of code size growing slightly.