strncmp: Fix issues related to very large n values.

This fixes the following issues:
*If n was 0x80000000 or greater, strncmp would return 0 without performing a comparison.
*If n was 0x1000000 or greater, strncmp might compare fewer characters than it should because the high byte of n was effectively ignored, causing it to return 0 when it should not.

Here is an example demonstrating these issues:

#pragma memorymodel 1
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#define LEN 100000
int main(void) {
        char *s1 = malloc(LEN+1);
        char *s2 = malloc(LEN+1);
        if (!s1 || !s2)
                return 0;
        for (unsigned long i = 0; i < LEN; i++) {
                s2[i] = s1[i] = '0' + (i & 0x07);
        }
        s1[LEN] = 'x';
        return strncmp(s1,s2,0xFFFFFFFF);
}

string.asm

@@ -1070,7 +1070,6 @@ flag     equ   1                        return flag
 
          ldy   #0                       scan until the end of string is reached
          ldx   n+2                       or a difference is found
-         bmi   equal
          bne   lb0
          ldx   n
          beq   equal
@@ -1082,9 +1081,11 @@ lb1      lda   [s1],Y
          bne   lb3
          dex
          bne   lb1a
-         lda   n+2
+         ldx   n+2
          beq   equal
-         dec   n+2
+         dex
+         stx   n+2
+         ldx   #0
 lb1a     iny
          bne   lb1
          inc   s1+2