xhci: fix uninitialized string returned by xhci_decode_ctrl_ctx()

xhci_decode_ctrl_ctx() returns the untouched buffer as-is if both "drop"
and "add" parameters are zero.

Fix the function to return an empty string in that case.

It was not immediately clear from the possible call chains whether this
issue is currently actually triggerable or not.

Note that before commit 4843b4b5ec ("xhci: fix even more unsafe memory
usage in xhci tracing") the result effect in the failure case was different
as a static buffer was used here, but the code still worked incorrectly.

Fixes: 90d6d5731d ("xhci: Add tracing for input control context")
Cc: stable@vger.kernel.org
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>

commit 4843b4b5ec ("xhci: fix even more unsafe memory usage in xhci tracing")
Link: https://lore.kernel.org/r/20220303110903.1662404-4-mathias.nyman@linux.intel.com

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.h

@@ -2468,6 +2468,8 @@ static inline const char *xhci_decode_ctrl_ctx(char *str,
 	unsigned int	bit;
 	int		ret = 0;
 
+	str[0] = '\0';
+
 	if (drop) {
 		ret = sprintf(str, "Drop:");
 		for_each_set_bit(bit, &drop, 32)