Fix riscv013_invalidate_cached_progbuf() off by one error

See https://github.com/riscv-collab/riscv-openocd/issues/1139
riscv013_invalidate_cached_progbuf() was failing to zeroize the final
buffer array element. Use memset() instead of a manual loop to zeroize
it in order to address this and simplify the code.

src/target/riscv/riscv-013.c

@@ -5282,8 +5282,7 @@ static int riscv013_invalidate_cached_progbuf(struct target *target)
 	}
 
 	LOG_TARGET_DEBUG(target, "Invalidating progbuf cache");
-	for (unsigned int i = 0; i < 15; i++)
+	memset(dm->progbuf_cache, 0, sizeof(dm->progbuf_cache));
-		dm->progbuf_cache[i] = 0;
 	return ERROR_OK;
 }