chore: Fix stack trace

os/src/entry.asm

@@ -3,6 +3,8 @@
 	.global _start
 _start:
 	la sp, boot_stack_top
+	# Make fp 0 so that stack trace knows where to stop
+	xor fp, fp, fp
 	j __kernel_start_main
 
 .section .bss.stack

os/src/stack_trace.rs

@@ -4,15 +4,17 @@ pub unsafe fn print_stack_trace() {
     let mut fp: *const usize;
     asm!("mv {}, fp", out(reg) fp);
 
+    let mut saved_ra = *fp.sub(1);
+    let mut saved_fp = *fp.sub(2);
+
     println!("== Begin stack trace ==");
-    while !fp.is_null() {
-        let saved_ra = *fp.sub(1);
-        let saved_fp = *fp.sub(2);
+    while !fp.is_null() && saved_fp != 0 {
+        saved_ra = *fp.sub(1);
+        saved_fp = *fp.sub(2);
 
         println!("0x{:016x}, fp = 0x{:016x}", saved_ra, saved_fp);
 
         fp = saved_fp as *const usize;
-
-        println!("== End stack trace ==");
     }
+    println!("== End stack trace ==");
 }