if the FileFd failed already following calls should fail, too

There is no point in trying to perform Write/Read on a FileFd which
already failed as they aren't going to work as expected, so we should
make sure that they fail early on and hard.

apt-pkg/contrib/fileutl.cc

@@ -2401,7 +2401,7 @@ FileFd::~FileFd()
    gracefully. */
 bool FileFd::Read(void *To,unsigned long long Size,unsigned long long *Actual)
 {
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return false;
    ssize_t Res = 1;
    errno = 0;
@@ -2452,7 +2452,7 @@ bool FileFd::Read(void *To,unsigned long long Size,unsigned long long *Actual)
 char* FileFd::ReadLine(char *To, unsigned long long const Size)
 {
    *To = '\0';
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return nullptr;
    return d->InternalReadLine(To, Size);
 }
@@ -2460,6 +2460,8 @@ char* FileFd::ReadLine(char *To, unsigned long long const Size)
 // FileFd::Flush - Flush the file  					/*{{{*/
 bool FileFd::Flush()
 {
+   if (Failed())
+      return false;
    if (d == nullptr)
       return true;
 
@@ -2469,7 +2471,7 @@ bool FileFd::Flush()
 // FileFd::Write - Write to the file					/*{{{*/
 bool FileFd::Write(const void *From,unsigned long long Size)
 {
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return false;
    ssize_t Res = 1;
    errno = 0;
@@ -2525,7 +2527,7 @@ bool FileFd::Write(int Fd, const void *From, unsigned long long Size)
 // FileFd::Seek - Seek in the file					/*{{{*/
 bool FileFd::Seek(unsigned long long To)
 {
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return false;
    Flags &= ~HitEof;
    return d->InternalSeek(To);
@@ -2534,7 +2536,7 @@ bool FileFd::Seek(unsigned long long To)
 // FileFd::Skip - Skip over data in the file				/*{{{*/
 bool FileFd::Skip(unsigned long long Over)
 {
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return false;
    return d->InternalSkip(Over);
 }
@@ -2542,7 +2544,7 @@ bool FileFd::Skip(unsigned long long Over)
 // FileFd::Truncate - Truncate the file					/*{{{*/
 bool FileFd::Truncate(unsigned long long To)
 {
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return false;
    // truncating /dev/null is always successful - as we get an error otherwise
    if (To == 0 && FileName == "/dev/null")
@@ -2555,7 +2557,7 @@ bool FileFd::Truncate(unsigned long long To)
 /* */
 unsigned long long FileFd::Tell()
 {
-   if (d == nullptr)
+   if (d == nullptr || Failed())
       return false;
    off_t const Res = d->InternalTell();
    if (Res == (off_t)-1)
@@ -2618,7 +2620,7 @@ time_t FileFd::ModificationTime()
 unsigned long long FileFd::Size()
 {
    if (d == nullptr)
-      return false;
+      return 0;
    return d->InternalSize();
 }
 									/*}}}*/