better error message when xsave context is too small

src/system/kernel/arch/x86/64/thread.cpp

@@ -144,7 +144,11 @@ arch_thread_init(kernel_args* args)
 	// Save one global valid FPU state; it will be copied in the arch dependent
 	// part of each new thread.
 	if (gHasXsave || gHasXsavec) {
-		ASSERT(gFPUSaveLength <= sizeof(sInitialState.fpu_state));
+		if (gFPUSaveLength > sizeof(sInitialState.fpu_state)) {
+			panic("XSAVE FPU context is larger than allowed "
+				"(need at least %" B_PRIu64 " bytes)", gFPUSaveLength);
+			return B_ERROR;
+		}
 		memset(sInitialState.fpu_state, 0, gFPUSaveLength);
 		if (gHasXsavec) {
 			asm volatile (