Fix an issue when reading blocks in streaming mode

[pnorbert]

when some producers did not write any block to output.

Issue could be reproduced before this fix by running localArray example with multiple processes. v3 is not written by every process and in the third step, there is a "gap" meaning, rank N did not write, but rank N+1 did write a block. Then the following script would fail in reading the blocks from the third step.

#!/usr/bin/env python3
import numpy as np
import adios2

def process_var(f, varname):
    print("--------------------------")
    print(f"Process variable {varname}")
    v = f.inquire_variable(varname)
    # b = f.all_blocks_info(varname)
    for step in range(v.steps()):
        v.set_step_selection([step, 1])
        bs = f.engine.blocks_info(varname, step)
        for bid in range(len(bs)):   # could use here len(b[step])
            v.set_block_selection(bid)
            data = f.read(v)
            print(f"step {step} block {bid} count = {data.shape}")


with adios2.Stream("localArray.bp", "r") as f:
    for _ in f.steps():
        process_var(f, 'v3')