bnxt_en: Fix null pointer dereference in bnxt_bs_trace_check_wrap()

With older FW, we may get the ASYNC_EVENT_CMPL_EVENT_ID_DBG_BUF_PRODUCER
for FW trace data type that has not been initialized.  This will result
in a crash in bnxt_bs_trace_type_wrap().  Add a guard to check for a
valid magic_byte pointer before proceeding.

Fixes: 84fcd94 ("bnxt_en: Manage the FW trace context memory")
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Shruti Parab <shruti.parab@broadcom.com>
Signed-off-by: Gautam R A <gautam-r.a@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20251104005700.542174-4-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Gautam R A

drivers/net/ethernet/broadcom/bnxt/bnxt.h

@@ -2149,7 +2149,7 @@ struct bnxt_bs_trace_info {
 static inline void bnxt_bs_trace_check_wrap(struct bnxt_bs_trace_info *bs_trace,
 					    u32 offset)
 {
-	if (!bs_trace->wrapped &&
+	if (!bs_trace->wrapped && bs_trace->magic_byte &&
 	    *bs_trace->magic_byte != BNXT_TRACE_BUF_MAGIC_BYTE)
 		bs_trace->wrapped = 1;
 	bs_trace->last_offset = offset;