Compiler: Fix runtime error in variadic functions that return strings.

Imported from SM-AM: 1d1244c2f0.

"This fixes a bug where returning a string from a variadic function caused
an invalid memory access error during runtime. It seems like they forgot
to update existing string return code for variadic functions."

compiler/libpc300/sc1.c

@@ -5288,6 +5288,22 @@ static symbol *fetchlab(char *name)
   return sym;
 }
 
+/* isvariadic
+ * 
+ * Checks if the function is variadic.
+ */
+static int isvariadic(symbol *sym)
+{
+  int i;
+  for (i=0; curfunc->dim.arglist[i].ident!=0; i++) {
+    /* check whether this is a variadic function */
+    if (curfunc->dim.arglist[i].ident==iVARARGS) {
+      return TRUE;
+    } /* if */
+  } /* for */
+  return FALSE;
+}
+
 /*  doreturn
  *
  *  Global references: rettype  (altered)
@@ -5392,7 +5408,19 @@ static void doreturn(void)
        * it stays on the heap for the moment, and it is removed -usually- at
        * the end of the expression/statement, see expression() in SC3.C)
        */
-      address(sub,sALT);                /* ALT = destination */
+        if (isvariadic(sub)) {
+          pushreg(sPRI);                  /* save source address stored in PRI */
+          sub->addr=2*sizeof(cell);
+          address(sub,sALT);              /* get the number of arguments */
+          getfrm();
+          addconst(3*sizeof(cell));
+          ob_add();
+          dereference();
+          swap1();
+          popreg(sALT);                   /* ALT = destination */
+        } else {
+          address(sub,sALT);              /* ALT = destination */
+        } /* if */
       arraysize=calc_arraysize(dim,numdim,0);
       memcopy(arraysize*sizeof(cell));  /* source already in PRI */
       /* moveto1(); is not necessary, callfunction() does a popreg() */