freopen improvements

This adds a check for the filename argument being null, in which case it bails out and returns NULL. Previously, it would just dereference the null pointer and treat the contents of memory location 0 as a filename, with unpredictable results. A null filename indicates freopen should try to reopen the same file with a different mode, but the degree of support for that is implementation-defined. We still don't really support it, but at least this will report the failure and avoid unpredictable behavior.

It also removes some unused code at the end, but still forces fputc and fgetc to be linked in. These are needed because there are weak references to them in putchar and getchar, which may need to be used if stdin/stdout have been reopened.

stdio.asm

@@ -1079,7 +1079,10 @@ cl1      ldy   #FILE_file               branch if the file is not a disk file
 ;
 ;  create a GS/OS file name
 ;
-cn1      ph4   filename                 get the length of the name buffer
+cn1      lda   filename                 bail out if filename is NULL
+         ora   filename+2                 (reopening same file not supported)
+         jeq   rt2
+         ph4   filename                 get the length of the name buffer
          jsl   ~osname
          sta   OSname
          stx   OSname+2
@@ -1259,33 +1262,9 @@ crFileType ds  2
 cl       dc    i'1'                     parameter block for OSclose
 clRefNum ds    2
 ;
-;  Patch for standard out
+;  Force inclusion of functions that have weak references elsewhere
 ;
-stdoutFile jmp stdoutPatch
-
-stdoutPatch phb
-         plx
-         ply
-         pla
-         pha
-         pha
-         pha
-         phy
-         phx
-         plb
-         lda   >stdout
-         sta   6,S
-         lda   >stdout+2
-         sta   8,S
-         brl   fputc
-;
-;  Patch for standard in
-;
-stdinFile jmp stdinPatch
-
-stdinPatch ph4 #stdin+4
-         jsl   fgetc
-         rtl
+         dc    r'fputc,fgetc'
          end
 
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