Fix zone allocator init on 1GB Pi 4

It was mistakenly assumed there would be a high heap available on the
1GB Raspberry Pi 4, which caused a boot failure on this model.

Check that there is a high heap available before attempting to allocate
from it, and fall back on the low heap if not.

CHANGELOG.md

@@ -34,6 +34,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - USB plug/unplug events now bring mt32-pi out of power saving mode.
 - Some USB MIDI devices that violate the USB specification were unusable (discussion #102) - a workaround in the USB driver has been implemented. Thanks to @fabbrimichele for reporting and @rsta2 for the fix!
 - Hang on "Init USB" when no USB controller is present (e.g. a Compute Module 4-based system with no external XHCI controller). Huge thanks to Serdaco for donating the CM4 and I/O board for testing!
+- Boot failure on the 1GB Raspberry Pi 4/CM4 because of a bug in the memory allocator.
 
 ## [0.9.1] - 2021-03-20
 

src/zoneallocator.cpp

@@ -27,6 +27,8 @@
 #include "utility.h"
 #include "zoneallocator.h"
 
+constexpr size_t MallocHeapSize = 32 * MEGABYTE;
+
 const char ZoneAllocatorName[] = "zoneallocator";
 
 // #define ZONE_ALLOCATOR_DEBUG
@@ -55,14 +57,22 @@ bool CZoneAllocator::Initialize()
 	CMemorySystem* pMemorySystem = CMemorySystem::Get();
 	CLogger* pLogger = CLogger::Get();
 
-#if RASPPI >= 4
-	// Allocate all of the remaining HIGH region
-	m_nHeapSize = pMemorySystem->GetHeapFreeSpace(HEAP_HIGH) - sizeof(THeapBlockHeader);
-	m_pHeap     = pMemorySystem->HeapAllocate(m_nHeapSize, HEAP_HIGH);
-#else
-	// Allocate the majority of the remaining LOW region
-	m_nHeapSize = pMemorySystem->GetHeapFreeSpace(HEAP_LOW) - 32 * MEGABYTE;
-	m_pHeap     = pMemorySystem->HeapAllocate(m_nHeapSize, HEAP_LOW);
+#if RASPI >= 4
+	const size_t nHighHeapSize = pMemorySystem->GetHeapFreeSpace(HEAP_HIGH);
+	if (nHighHeapSize)
+	{
+		// >1GB RAM Pi 4 - allocate all of the remaining HIGH region
+		m_nHeapSize = nHighHeapSize - sizeof(THeapBlockHeader);
+		m_pHeap     = pMemorySystem->HeapAllocate(m_nHeapSize, HEAP_HIGH);
+	}
+	else
+	{
+#endif
+		// Allocate the majority of the remaining LOW region; leave some space for Circle/libc malloc()
+		m_nHeapSize = pMemorySystem->GetHeapFreeSpace(HEAP_LOW) - MallocHeapSize;
+		m_pHeap     = pMemorySystem->HeapAllocate(m_nHeapSize, HEAP_LOW);
+#if RASPI >= 4
+	}
 #endif
 
 	if (!m_pHeap)