fix oob read on empty block member list in getBlockSize

[saddamr3e]

ASAN, compute shader with a forward-declared GL_EXT_buffer_reference block that never gets a body:

SEGV on unknown address 0xffffffffffffffe0
#0 glslang::TIntermediate::getOffset(TType const&, int) linkValidate.cpp:2736
#1 glslang::TIntermediate::getBlockSize(TType const&) linkValidate.cpp:2757
#2 glslang::TIntermediate::computeBufferReferenceTypeSize(TType const&) linkValidate.cpp:2771
#3 glslang::TIntermediate::addBinaryMath(...) Intermediate.cpp:136

getBlockSize takes lastIndex = memberList.size() - 1 and indexes memberList[lastIndex]. The referent of a forward-declared buffer_reference whose block body never appears keeps an empty member list, so lastIndex is -1 and TVector::operator[] turns that into a huge size_t offset. Pointer math on such a reference (b = b + 1, or b[0]) reaches it. An empty block is 0 bytes, so return 0 before the subtraction.

Test: spv.bufferReferenceForwardDecl.comp.