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/flash/nor/jtagspi.c

@@ -59,7 +59,7 @@ static void jtagspi_set_ir(struct flash_bank *bank)
 {
 	struct jtagspi_flash_bank *info = bank->driver_priv;
 	struct scan_field field;
-	uint8_t buf[4];
+	uint8_t buf[4] = { 0 };
 
 	LOG_DEBUG("loading jtagspi ir");
 	buf_set_u32(buf, 0, info->tap->ir_length, info->ir);