one null pointer deference bug in AP4_JsonInspector::AddField in Ap4Atom.cpp

[92wyunchao]

To reproduce:
./mp4dump --format json $poc
poc:
poc.zip

ASan:
==80662==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000005e2b1f bp 0x7fff14fa2cb0 sp 0x7fff14fa2aa0 T0)
#0 0x5e2b1e in AP4_JsonInspector::AddField(char const*, unsigned char const*, unsigned int, AP4_AtomInspector::FormatHint) /home/s2e/1/Bento4-master/Source/C++/Core/Ap4Atom.cpp:1166
#1 0x586bc7 in AP4_PsshAtom::InspectFields(AP4_AtomInspector&) /home/s2e/1/Bento4-master/Source/C++/Core/Ap4PsshAtom.cpp:238
#2 0x5d8b24 in AP4_Atom::Inspect(AP4_AtomInspector&) /home/s2e/1/Bento4-master/Source/C++/Core/Ap4Atom.cpp:263
#3 0x5312ec in AP4_AtomListInspector::Action(AP4_Atom*) const /home/s2e/1/Bento4-master/Source/C++/Core/Ap4Atom.h:528
#4 0x52f8b6 in AP4_List<AP4_Atom>::Apply(AP4_List<AP4_Atom>::Item::Operator const&) const /home/s2e/1/Bento4-master/Source/C++/Core/Ap4List.h:353
#5 0x52f8b6 in AP4_ContainerAtom::InspectChildren(AP4_AtomInspector&) /home/s2e/1/Bento4-master/Source/C++/Core/Ap4ContainerAtom.cpp:220
#6 0x5d8b24 in AP4_Atom::Inspect(AP4_AtomInspector&) /home/s2e/1/Bento4-master/Source/C++/Core/Ap4Atom.cpp:263
#7 0x527f9e in main /home/s2e/1/Bento4-master/Source/C++/Apps/Mp4Dump/Mp4Dump.cpp:350
#8 0x7fc5c6b8982f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291
#9 0x450e48 in _start (/home/s2e/1/Bento4-master/cmakebuild/mp4dump+0x450e48)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/s2e/1/Bento4-master/Source/C++/Core/Ap4Atom.cpp:1166 in AP4_JsonInspector::AddField(char const*, unsigned char const*, unsigned int, AP4_AtomInspector::FormatHint)
==80662==ABORTING