malloc size in assembleBlockAndSubmit

[jesterhodl]

There's a safety provision in assembleBlockAndSubmit() to use a hardcoded value of 8500000:

	// each thread has a chunk of RAM dedicated to prepping block submissions. use it.
	submitblock_req = sdata->submitblock_req;
	
	if (!submitblock_req) {
		// This should NEVER happen and likely indicates something is terribly wrong with the state of things... but we'll try our best to salvage this block.
		DLOG_ERROR("For some reason no pointer available for submitting the block we just found! Attempting to allocate new memory for this, but we're probably in for a bad time...");
		submitblock_req = malloc(8500000); // worst case
...

Shouldn't we align with the defined value and use: MAX_SUBMITBLOCK_SIZE instead?