sysfs.c: fix an theoretical issue with snprintf()

Again, code scanners really don't like C string functions, and warn
about the potential for a buffer to possibly be too big for a snprintf()
call as the checking wasn't quite correct.  Fix the check for a buffer
overflow up better and handle any potential issues that checking tools
could come up with.

No real functional change as the sysfs path should be just fine.  And
really, this is all just userspace code...

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

sysfs.c

@@ -44,8 +44,12 @@ int get_sysfs_name(char *buf, size_t size, libusb_device *dev)
 	}
 
 	len += snprintf(buf, size, "%d-", bnum);
-	for (int i = 0; i < num_pnums; i++)
-		len += snprintf(buf + len, size - len, i ? ".%d" : "%d", pnums[i]);
+	for (int i = 0; i < num_pnums; i++) {
+		int n = snprintf(buf + len, size - len, i ? ".%d" : "%d", pnums[i]);
+		if ((n < 0) || (n >= (int)(size - len)))
+			break;
+		len += n;
+	}
 
 	return len;
 }