malloc: Skip backtrace when stack is corrupted

When the stack is corrupted (e.g., by the stack protector test),
collecting a backtrace during malloc causes a crash because the
backtrace code walks the invalid stack frames.

Update __stack_chk_fail() to set the flag before calling panic()

Also update stackprot_test() to set the flag before intentionally
corrupting the stack. This is needed because of the printf() in the
test: on sandbox printf() results in truetype allocations due to the
console output.

These fixes allow the stack protector test to pass with mcheck enabled.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

common/stackprot.c

@@ -4,6 +4,7 @@
  */
 
 #include <asm/global_data.h>
+#include <malloc.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
@@ -13,6 +14,11 @@ void __stack_chk_fail(void)
 {
 	void *ra;
 
+	/*
+	 * When the stack is corrupted, backtrace collection will crash.
+	 * Skip it before calling panic().
+	 */
+	malloc_backtrace_skip(true);
 	ra = __builtin_extract_return_addr(__builtin_return_address(0));
 	panic("Stack smashing detected in function:\n%p relocated from %p",
 	      ra, ra - gd->reloc_off);