binarybuffer: Fix inverted return value in buf_cmp

This is the fast path for when there is a mismatch in the leading whole
bytes, which means we should return true to indicate not equal like all
the other cases here and in the surrounding functions. Otherwise we'll
incorrectly report _buf1 == _buf2 if and only if there are mismatches in
the leading whole bytes.

This was introduced during the refactor and optimisation referenced
below.

The only in-tree caller of this is jtag_check_value_inner, which will
just fail to catch some errors. However, downstream in riscv-openocd it
gets used in the riscv target to determine whether an IR scan is needed
to select the debug module, and with an IRLEN >= 8 this breaks resetting
if the encoding for the DMI isn't all-ones in its leading whole bytes
(to match BYPASS), since it will believe they are the same and not do an
IR scan, failing (with "At least one TAP shouldn't be in BYPASS mode")
in the subsequent DR scan due to the TAP still being recorded as having
bypass set (and really having an instruction of either BYPASS or
IDCODE).

Fixes: e4ee891 ("improve buf_cmp and buf_cmp_mask helpers")
Change-Id: Ic4f7ed094429abc4c06a775eb847a8b3ddf2e2d6
Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8489
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins

src/helper/binarybuffer.c

@@ -74,7 +74,7 @@ bool buf_cmp(const void *_buf1, const void *_buf2, unsigned size)
 
 	unsigned last = size / 8;
 	if (memcmp(_buf1, _buf2, last) != 0)
-		return false;
+		return true;
 
 	unsigned trailing = size % 8;
 	if (!trailing)