Fix KBASE for Lua functions below stitched fast functions.

src/vm_x86.dasc

@@ -2697,6 +2697,8 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov RB, TMP1			// Get previous trace number.
   |  mov RA, [DISPATCH+DISPATCH_J(trace)]
   |  mov TRACE:RD, [RA+RB*4]
+  |  test TRACE:RD, TRACE:RD
+  |  jz ->cont_nop
   |  movzx RD, word TRACE:RD->link
   |  cmp RD, RB
   |  je ->cont_nop			// Blacklisted.
@@ -2844,7 +2846,7 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov r13, TMPa
   |  mov r12, TMPQ
   |.endif
-  |  test RD, RD; js >3			// Check for error from exit.
+  |  test RD, RD; js >9			// Check for error from exit.
   |  mov L:RB, SAVE_L
   |  mov MULTRES, RD
   |  mov LFUNC:KBASE, [BASE-8]
@@ -2860,16 +2862,31 @@ static void build_subroutines(BuildCtx *ctx)
   |  add PC, 4
   |  shr RC, 16
   |  cmp OP, BC_FUNCF			// Function header?
-  |  jb >2
-  |  mov RC, MULTRES			// RC/RD holds nres+1.
+  |  jb >3
+  |  cmp OP, BC_FUNCC+2			// Fast function?
+  |  jae >4
   |2:
+  |  mov RC, MULTRES			// RC/RD holds nres+1.
+  |3:
   |.if X64
   |  jmp aword [DISPATCH+OP*8]
   |.else
   |  jmp aword [DISPATCH+OP*4]
   |.endif
   |
-  |3:  // Rethrow error from the right C frame.
+  |4:  // Check frame below fast function.
+  |  mov RC, [BASE-4]
+  |  test RC, FRAME_TYPE
+  |  jnz <2				// Trace stitching continuation?
+  |  // Otherwise set KBASE for Lua function below fast function.
+  |  movzx RC, byte [RC-3]
+  |  not RCa
+  |  mov LFUNC:KBASE, [BASE+RC*8-8]
+  |  mov KBASE, LFUNC:KBASE->pc
+  |  mov KBASE, [KBASE+PC2PROTO(k)]
+  |  jmp <2
+  |
+  |9:  // Rethrow error from the right C frame.
   |  neg RD
   |  mov FCARG1, L:RB
   |  mov FCARG2, RD