Fix bug causing data corruption when assigning to multiple structs.

This affects code where multiple structs or unions are assigned by successive = operators in one expression, e.g. "s1=s2=s3". The middle struct assignment(s) would use the ~Move2 or ~LongMove2 helper functions (for <64k or >=64k moves, respectively). These functions are supposed to leave the destination pointer on the stack so it can be used as the source of a subsequent move, but they both had bugs where they could modify dest and leave that modified value on the stack, which would cause subsequent moves to use the wrong source location. In the case of ~Move2, this only happened if the size was odd.

Here is a program that demonstrated the problems with both functions:

#pragma memorymodel 1
#include <stdio.h>

struct S1 {
        char s[80000];
} a,b,c;

int main(void) {
        struct S2 {
                int x,y;
                char z;
        } d,e,f;

        c.s[66000] = 123;
        f.y = 5678;

        a = b = c;
        d = e = f;

        printf("%i %i %i\n", a.s[66000], b.s[66000], c.s[66000]);
        printf("%i %i %i\n", d.y, e.y, f.y);
}

cc.asm

@@ -727,6 +727,8 @@ lb3      sec
          csubroutine (4:len,4:source),0
 dest     equ   source+4
 
+         pei   dest+2                   save original dest value
+         pei   dest
          ldx   len+2                    move whole banks
          beq   lm2
          ldy   #0
@@ -761,7 +763,11 @@ lb2      lda   [source],Y
          bne   lb2
 lb3      lda   [source]
          sta   [dest]
-lb4      creturn
+lb4      pla                            restore original dest value
+         sta   dest
+         pla
+         sta   dest+2
+         creturn
          end
 
 ****************************************************************
@@ -862,7 +868,9 @@ lb2      lda   [source],Y
          bne   lb2
 lb3      lda   [source]
          sta   [dest]
-lb4      creturn
+lb4      bcc   lb5                      if the move length was odd
+         dec4  dest                       restore original dest value
+lb5      creturn
          end
 
 ****************************************************************

cc.macros

@@ -280,11 +280,6 @@
 ~&SYSCNT ~RESTM
  MEND
  MACRO
-&LAB JEQ &BP
-&LAB BNE *+5
- BRL &BP
- MEND
- MACRO
 &LAB LONG &A,&B
  LCLB &I
  LCLB &M
@@ -539,3 +534,12 @@
 .j
  rtl
  mend
+ macro
+&l dec4 &a
+&l ~setm
+ lda &a
+ bne ~&SYSCNT
+ dec 2+&a
+~&SYSCNT dec &a
+ ~restm
+ mend