helper/binarybuffer: fix clang static analyzer warnings

Writing bits to an uninitialized buffer generated false warnings.
Zero buffers before setting them by buf_set_u32|64()
(do it only if bit-by-bit copy loop is used,
zeroed buffer is not necessary if a fast path write is used)

Change-Id: I2f7f8ddb45b0cbd08d3e249534fc51f4b5cc6694
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5383
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

src/target/riscv/riscv-013.c

@@ -402,7 +402,7 @@ static uint32_t dtmcontrol_scan(struct target *target, uint32_t out)
 {
 	struct scan_field field;
 	uint8_t in_value[4];
-	uint8_t out_value[4];
+	uint8_t out_value[4] = { 0 };
 
 	buf_set_u32(out_value, 0, 32, out);
 
@@ -468,6 +468,7 @@ static dmi_status_t dmi_scan(struct target *target, uint32_t *address_in,
 	}
 
 	memset(in, 0, num_bytes);
+	memset(out, 0, num_bytes);
 
 	assert(info->abits != 0);