fix out-of-bounds read on empty block in layoutObjectCheck

[saddamr3e]

ASan, Vulkan/SPIR-V target, tessellation-control stage:

ParseHelper.cpp:7770: container-overflow ... layoutObjectCheck
READ of size 8 ... (*type.getStruct())[0]

Redeclaring a built-in block with an instance name and members that don't match
the originals erases every original member (the instance-name erase in
redeclareBuiltinBlock), so the block is left with an empty member list.
layoutObjectCheck then reads member [0] to inspect its location qualifier, past
the end of an empty list. Skip the member check when the block has no members.

Repro:

#version 450
in gl_PerVertex {
    float bad;
} gl_in[];
void main() {}

compiled for a tesc stage with a Vulkan target.

[saddamr3e]

gentle ping