Fix bug where ftell would inappropriately change the file mark.

ftell would set the mark as if the buffer and putback were being discarded, but not actually discard them. This resulted in characters being reread once the end of the buffer was reached.

Here is an example that illustrates the problem:

#include <stdio.h>
#include <string.h>
char buf[100];
int main(void) {
        FILE *f = fopen("somefile","r"); // a file with some text
        if (!f) return 0;
        setvbuf(f, 0L, _IOFBF, 14); // to see problem sooner
        fgets(buf, 10, f);
        printf("first read : %s\n", buf);
        ftell(f);
        memset(buf, 0, sizeof(buf));
        fgets(buf, 10, f);
        printf("second read: %s\n", buf);]
}

stdio.asm

@@ -378,9 +378,19 @@ fl1a     bit   #_IORW
          beq   fl3
          bit   #_IOREAD                   if the file is being read then
          beq   fl2
-         ph4   <stream                      use ftell to set the mark
+         ph4   <stream                      set the mark to current position
          jsl   ftell
-         ldy   #FILE_flag
+         cmp   #-1
+         bne   fl1b
+         cpx   #-1
+         beq   fl1c
+fl1b     sta   smPosition
+         stx   smPosition+2
+         ldy   #FILE_file
+         lda   [stream],Y
+         sta   smRefNum
+         OSSet_Mark sm
+fl1c     ldy   #FILE_flag
          lda   [stream],Y
 fl2      and   #$FFFF-_IOWRT-_IOREAD      turn off the reading and writing flags
          sta   [stream],Y
@@ -396,6 +406,11 @@ wrDataBuffer ds 4
 wrRequestCount ds 4
          ds    4
          dc    i'1'
+
+sm       dc    i'3'                     parameter block for OSSet_Mark
+smRefNum ds    2
+         dc    i'0'
+smPosition ds  4
          end
 
 ****************************************************************
@@ -2014,26 +2029,16 @@ lb1      move4 gmPosition,pos           set the position
          sta   pos+2
          ldy   #FILE_pbk                  dec pos by 1 for each char in the
          lda   [stream],Y                   putback buffer then
-         bmi   lb2
+         bmi   rts
          dec4  pos
          ldy   #FILE_pbk+2
          lda   [stream],Y
-         bmi   lb2
+         bmi   rts
          dec4  pos
-lb2      ldy   #FILE_file                 set the file's mark
-         lda   [stream],Y
-         sta   spRefNum
-         move4 pos,spPosition
-         OSSet_Mark sp
 
 rts      plb
          creturn 4:pos
 
-sp       dc    i'3'                     parameter block for OSSet_Mark
-spRefNum ds    2
-         dc    i'0'
-spPosition ds  4
-
 gm       dc    i'2'                     parameter block for OSGetMark
 gmRefNum ds    2
 gmPosition ds  4