Fix panic when size 0 passed to malloc

src/runtime/arch_tinygowasm_malloc.go

@@ -13,6 +13,9 @@ var allocs = make(map[uintptr][]byte)
 
 //export malloc
 func libc_malloc(size uintptr) unsafe.Pointer {
+	if size == 0 {
+		return nil
+	}
 	buf := make([]byte, size)
 	ptr := unsafe.Pointer(&buf[0])
 	allocs[uintptr(ptr)] = buf
@@ -39,6 +42,11 @@ func libc_calloc(nmemb, size uintptr) unsafe.Pointer {
 
 //export realloc
 func libc_realloc(oldPtr unsafe.Pointer, size uintptr) unsafe.Pointer {
+	if size == 0 {
+		libc_free(oldPtr)
+		return nil
+	}
+
 	// It's hard to optimize this to expand the current buffer with our GC, but
 	// it is theoretically possible. For now, just always allocate fresh.
 	buf := make([]byte, size)

tests/runtime_wasi/malloc_test.go

@@ -125,3 +125,32 @@ func TestMallocFree(t *testing.T) {
 		})
 	}
 }
+
+func TestMallocEmpty(t *testing.T) {
+	ptr := libc_malloc(0)
+	if ptr != nil {
+		t.Errorf("expected nil pointer, got %p", ptr)
+	}
+}
+
+func TestCallocEmpty(t *testing.T) {
+	ptr := libc_calloc(0, 1)
+	if ptr != nil {
+		t.Errorf("expected nil pointer, got %p", ptr)
+	}
+	ptr = libc_calloc(1, 0)
+	if ptr != nil {
+		t.Errorf("expected nil pointer, got %p", ptr)
+	}
+}
+
+func TestReallocEmpty(t *testing.T) {
+	ptr := libc_malloc(1)
+	if ptr == nil {
+		t.Error("expected pointer but was nil")
+	}
+	ptr = libc_realloc(ptr, 0)
+	if ptr != nil {
+		t.Errorf("expected nil pointer, got %p", ptr)
+	}
+}