check osthreadstate is not null before dereferencing

[richardstartin]

What does this PR do?:
In the original bug report, we had the following information:

Current thread (0x00007f665b5ef090):  Thread [stack: 0x00007f661685f000,0x00007f661695fa90] [id=0]

Stack: [0x00007f661685f000,0x00007f661695fa90],  sp=0x00007f661695ef30,  free space=1023k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
C  [libjavaProfiler1675892613987672088.so+0x39634]  WallClock::signalHandler(int, siginfo_t*, void*, unsigned long long)+0x244


siginfo: si_signo: 11 (SIGSEGV), si_code: 1 (SEGV_MAPERR), si_addr: 0x00007f6632402020

Register to memory mapping:

RAX=0x00007f6632402010 points into unknown readable memory: 0x0000000000000000 | 00 00 00 00 00 00 00 00

Which shows that 0x00007f6632402010 (const char* osthread) points to null, so adding the thread state offset (_osthread_state_offset) to get 0x00007f6632402020 would produce a segfault when dereferencing. Instead of using SafeAccess::load32, which causes obscure crashes, we can check the pointer, but also check if the thread is even initialized before trying to access the OSThread* to make avoiding crashing here less racy.

Motivation:

Additional Notes:

How to test the change?:

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

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🔧 Report generated by pr-comment-cppcheck

CppCheck Report

Warnings (5)

• Obsolete function 'alloca' called.
• Member variable 'ScopeDesc::_stream' is not initialized in the constructor.
• Member variable 'ScopeDesc::_method_offset' is not initialized in the constructor.
• Member variable 'ScopeDesc::_bci' is not initialized in the constructor.
• Member variable 'ElfParser::_vaddr_diff' is not initialized in the constructor.

Style Violations (164)

• Local variable 'jni' shadows outer function
• Local variable 'size' shadows outer function
• Local variable 'prev' shadows outer function
• The class 'NativeFunc' does not declare a constructor although it has private member variables which likely require initialization.
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• Variable 'error' is assigned a value that is never used.
• Variable 'error' is assigned a value that is never used.
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• Unused private function: 'StackFrame::withinCurrentStack'
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• The comparison 'EMPTY_FRAME_SIZE > 0' is always false.
• The scope of the variable 'prev_sp' can be reduced.
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🔧 Report generated by pr-comment-scanbuild

Scan-Build Report

User:	runner@fv-az714-534
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 clean all
Clang Version:	Ubuntu clang version 14.0.0-1ubuntu1.1
Date:	Wed Feb 28 14:37:52 2024

Bug Summary

Bug Type	Quantity	Display?
All Bugs	6
Logic error
Assigned value is garbage or undefined	1
Dereference of null pointer	2
Result of operation is garbage or undefined	1
Unused code
Dead initialization	1
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Assigned value is garbage or undefined	dwarf.cpp	parseInstructions	235	20
Unused code	Dead initialization	livenessTracker.cpp	cleanup_table	45	1
Unused code	Dead nested assignment	vmStructs.cpp	checkNativeBinding	818	1
Logic error	Dereference of null pointer	flightRecorder.cpp	flush	1410	8
Logic error	Dereference of null pointer	safeAccess.h	load	35	18
Logic error	Result of operation is garbage or undefined	dwarf.h	getSLeb	142	25