sandbox: disable tracing before unmapping RAM

Currently doing 'reset' command in sandbox with tracing enabled causes
SIGSEV

```
Hit any key to stop autoboot:  0
=>
=>
=> reset
resetting ...
Segmentation fault (core dumped)

```

Tracing callback uses RAM buffer for storing tracing reports, but
state_uninit() function unmaps whole RAM, which causes SIGSEV on umapped
memory inside tracing subsystem.

Fix it by disabling tracing before unmapping memory

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Add missing check for CONFIG_TRACE:
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/state.c b/arch/sandbox/cpu/state.c
index 69da378..d678349 100644
--- a/arch/sandbox/cpu/state.c
+++ b/arch/sandbox/cpu/state.c
@@ -10,6 +10,7 @@
 #include <fdtdec.h>
 #include <log.h>
 #include <os.h>
+#include <trace.h>
 #include <asm/malloc.h>
 #include <asm/state.h>
 #include <asm/test.h>
@@ -525,6 +526,10 @@
 	if (state->jumped_fname)
 		os_unlink(state->jumped_fname);
 
+	/* Disable tracing before unmapping RAM */
+	if (IS_ENABLED(CONFIG_TRACE))
+		trace_set_enabled(0);
+
 	os_free(state->state_fdt);
 	os_free(state->ram_buf);
 	memset(state, '\0', sizeof(*state));