virCommand: Make virCommandPassFDGetFDIndex fail if passed command is in error state

The idea of virCommand* APIs is that a possible error that
occurred while constructing cmd line is kept in virCommand
struct. If that's the case all subsequent calls to virCommand*()
are NO-OPs or they return an error. Well,
virCommandPassFDGetFDIndex() is not honoring that.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

src/util/vircommand.c

@@ -1034,6 +1034,9 @@ virCommandPassFDGetFDIndex(virCommandPtr cmd, int fd)
 {
     size_t i = 0;
 
+    if (!cmd || cmd->has_error)
+        return -1;
+
     while (i < cmd->npassfd) {
         if (cmd->passfd[i].fd == fd)
             return i;