strncat: fix more issues related to large n values.

This addresses the following issues:
*If the low-order 16 bits of n were 0x0000, no concatenation would be performed.
*If n was 0x1000000 or greater, the output could be cut off prematurely because the high byte of n was effectively ignored.

The following test program demonstrates these issues:

#pragma memorymodel 1
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#define LEN2 100000
int main(void) {
        char *s1 = malloc(LEN2+2);
        char *s2 = malloc(LEN2+1);
        if (!s1 || !s2)
                return 0;
        for (unsigned long i = 0; i < LEN2; i++)
                s2[i] = '0' + (i & 0x07);
        strcpy(s1,"a");
        strncat(s1, s2, 0x1000000);
        puts(s1);
        printf("len = %zu\n", strlen(s1));
}

string.asm

@@ -1017,29 +1017,33 @@ lb2      long  M
          sta   s1
          bcc   lb2a
          inc   s1+2
-lb2a     short M                        copy characters 'til the null is found
+lb2a     ldx   n                        copy characters 'til the null is found
+         bne   lb2b
+         lda   n+2
+         beq   lb6
+lb2b     short M
          ldy   #0
-         ldx   n
-         beq   lb4
 lb3      lda   [s2],Y
          sta   [s1],Y
-         beq   lb4
+         beq   lb5
          iny
          bne   lb3a
          inc   s1+2
          inc   s2+2
 lb3a     dex
          bne   lb3
-         lda   n+2
+         ldx   n+2
          beq   lb4
-         dec   n+2
+         dex
+         stx   n+2
+         ldx   #0
          bra   lb3
 
 lb4      lda   #0                       write the terminating null
          sta   [s1],Y
-         long  M                        return to the caller
+lb5      long  M                        return to the caller
 
-         creturn 4:rval
+lb6      creturn 4:rval
          end
 
 ****************************************************************