RISC-V: Fix testcase for vector .SAT_SUB in zip benchmark

RISC-V: Fix testcase for vector .SAT_SUB in zip benchmark

The following testcase was not properly testing anything due to an
uninitialized variable. As a result, the loop was not iterating through
the testing data, but instead on undefined values which could cause an
unexpected abort.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/vec_sat_binary_vx.h:
	initialize variable

Signed-off-by: Edwin Lu <ewlu@rivosinc.com>