Potential `nullptr` dereference in LLVM to SPIR-V Debug info translation

[AlexeySachkov]

In the following code BB can be set to nullptr, but it is unconditionally dereferenced later which could lead to segfaults

SPIRV-LLVM-Translator/lib/SPIRV/LLVMToSPIRVDbgTran.cpp

Lines 1351 to 1355 in cb6891b

	SPIRVBasicBlock *BB = F->getNumBasicBlock() ? F->getBasicBlock(0) : nullptr;
	SPIRVId ExtSetId = BM->getExtInstSetId(BM->getDebugInfoEIS());
	return BM->addExtInst(getVoidTy(), ExtSetId, SPIRVDebug::FunctionDefinition,
	Ops, BB, BB->getInst(0));