Used by the routine at START.

Called once, by START. It loads HL with 0 and rotates R's top bit into the carry, and nothing uses either. With the byte after it, JP_HL_LEFTOVER, a JP (HL), it reads like the remains of a check that jumped to address 0 -- a reset -- when bit 7 of R was set, which only an LD R,A can do; with a plain RET where a conditional return would be, it never jumps.