Fix assertion failure in IdealGraphPrinter when append is true

When `IdealGraphPrinter` is created with `append` set to true, it does not write the head element to the file in its constructor. But in the destructor, it always writes the tail element to the file no matter `append` is true or not, which causes an assertion failure in `xmlStream::pop_tag`:

# Internal Error (/root/jdk/src/hotspot/share/utilities/xmlstream.cpp:186), pid=1520448, tid=1520452
# assert(_element_depth > 0) failed: must be in an element to close

Stack: [0x00007ffff59fc000,0x00007ffff5afd000], sp=0x00007ffff5afbc70, free space=1023k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x197e9d3] xmlStream::pop_tag(char const*)+0x1c3 (xmlstream.cpp:186)
V [libjvm.so+0x197f8c3] xmlStream::tail(char const*)+0x13 (xmlstream.cpp:317)
V [libjvm.so+0xdce4ec] IdealGraphPrinter::~IdealGraphPrinter()+0x1c (idealGraphPrinter.cpp:218)
V [libjvm.so+0xdce5ff] IdealGraphPrinter::clean_up()+0x9f (idealGraphPrinter.cpp:106)
V [libjvm.so+0x17d03c7] Threads::destroy_vm()+0x267 (threads.cpp:949)
V [libjvm.so+0xf7571c] jni_DestroyJavaVM+0x9c (jni.cpp:3733)
C [libjli.so+0x455f] JavaMain+0x2bf (java.c:679)
C [libjli.so+0x7cc9] ThreadJavaMain+0x9 (java_md.c:653)

This issue can be reproduced by calling `igv_append` directly, or by calling `Compile::igv_print_method_to_file` with `append` set to true.

This patch fixes the issue.