fix(wasi) do not use instance pool in 'fd_write'

Avoid using the long-lived instance pool for a temporary buffer on
fd_write. Since we allocate and deallocate deterministically within the
same function (and to avoid a cross-concern dependency on proxy-wasm to
get pwctx), we simply use ngx_alloc and ngx_free to produce a temporary
buffer.

src/wasm/wasi/ngx_wasi_host.c

@@ -258,7 +258,11 @@ ngx_wasi_hfuncs_fd_write(ngx_wavm_instance_t *instance,
 
     /* second pass: build string from iov buffers */
 
-    msg_data = ngx_pcalloc(instance->pool, msg_size);
+    /**
+     * TODO: replace with a host-pointer-provided pool when possible;
+     * see https://github.com/Kong/ngx_wasm_module/pull/457
+     */
+    msg_data = ngx_alloc(msg_size, instance->log);
     if (msg_data == NULL) {
         rets[0] = (wasm_val_t) WASM_I32_VAL(WASI_ERRNO_EFAULT);
         return NGX_WAVM_OK;
@@ -291,6 +295,8 @@ ngx_wasi_hfuncs_fd_write(ngx_wavm_instance_t *instance,
                       msg_size, msg_data);
     }
 
+    ngx_free(msg_data);
+
     rets[0] = (wasm_val_t) WASM_I32_VAL(WASI_ERRNO_SUCCESS);
     return NGX_WAVM_OK;
 }