esirisc_jtag: fix clang error core.VLASize

esirisc_jtag: fix clang error core.VLASize

The function esirisc_jtag_recv() can be called with argument
num_in_fields = 0, for example as consequence of calling
esirisc_jtag_continue().
In this case, num_in_bytes is zero and the allocation of the
variable-length array 'r' requires size zero.

  src/target/esirisc_jtag.c:133:2: warning: Declared variable-length
    array (VLA) has zero size [core.VLASize]
        uint8_t r[num_in_bytes * 2];
        ^~~~~~~~~ ~~~~~~~~~~~~~~~~

Fix it by forcing size one when num_in_bytes is zero.

Change-Id: Id764c7b5ec4f5b3c18c7da650bbff39fc98ed049
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7301
Tested-by: jenkins