fix(kernel): avoid allocating 0-length blocks (#465)

Fixes an issue where the kernel was able to allocate empty blocks,
causing some weird memory issues.

kernel/src/lib.rs

@@ -310,6 +310,9 @@ impl MemoryBlock {
 /// Allocate a block of memory and return the offset
 #[no_mangle]
 pub unsafe fn extism_alloc(n: Length) -> Pointer {
+    if n == 0 {
+        return 0;
+    }
     let region = MemoryRoot::new();
     let block = region.alloc(n);
     match block {
@@ -321,6 +324,9 @@ pub unsafe fn extism_alloc(n: Length) -> Pointer {
 /// Free allocated memory
 #[no_mangle]
 pub unsafe fn extism_free(p: Pointer) {
+    if p == 0 {
+        return;
+    }
     let block = MemoryRoot::new().find_block(p);
     if let Some(block) = block {
         block.free();

runtime/src/current_plugin.rs

@@ -125,6 +125,12 @@ impl CurrentPlugin {
     }
 
     pub fn memory_alloc(&mut self, n: u64) -> Result<MemoryHandle, Error> {
+        if n == 0 {
+            return Ok(MemoryHandle {
+                offset: 0,
+                length: 0,
+            });
+        }
         let (linker, mut store) = self.linker_and_store();
         let output = &mut [Val::I64(0)];
         if let Some(f) = linker.get(&mut store, "env", "extism_alloc") {