Improve zlib test and ImmutableBinaryBlob API

backend.native/tests/interop/platform_zlib.kt

@@ -1,21 +1,22 @@
 import kotlinx.cinterop.*
+
 import platform.zlib.*
 
-val source = immutableBinaryBlobOf(0xF3, 0x48, 0xCD, 0xC9, 0xC9, 0x57, 0x04, 0x00).asCPointer(0)!!
-val golden = immutableBinaryBlobOf(0x48, 0x65, 0x6C, 0x6C, 0x6F, 0x21, 0x00).asCPointer(0)!!
+val source = immutableBinaryBlobOf(0xF3, 0x48, 0xCD, 0xC9, 0xC9, 0x57, 0x04, 0x00).asCPointer()
+val golden = immutableBinaryBlobOf(0x48, 0x65, 0x6C, 0x6C, 0x6F, 0x21, 0x00).asCPointer()
 
 fun main(args: Array<String>) = memScoped {
-    val buffer = allocArray<ByteVar>(32)
-
-    val z = alloc<z_stream>().apply {
-        next_in   = source
-        avail_in  = 8
-        next_out  = buffer
-        avail_out = 32
-    }.ptr
-
-    if (inflateInit2(z, -15) == Z_OK && inflate(z, Z_FINISH) == Z_STREAM_END && inflateEnd(z) == Z_OK) 
-        println(buffer.toKString())
+    val buffer = ByteArray(32)
+    buffer.usePinned { pinned ->
+        val z = alloc<z_stream>().apply {
+            next_in = source
+            avail_in = 8
+            next_out = pinned.addressOf(0)
+            avail_out = buffer.size
+        }.ptr
 
+        if (inflateInit2(z, -15) == Z_OK && inflate(z, Z_FINISH) == Z_STREAM_END && inflateEnd(z) == Z_OK)
+            println(buffer.stringFromUtf8())
+    }
     println(golden.toKString())
 }

runtime/src/main/kotlin/kotlin/native/BinaryBlob.kt

@@ -61,7 +61,8 @@ public fun ImmutableBinaryBlob.toByteArray() = toByteArray(0, size)
 
 // Returns stable C pointer to data at certain offset, useful as a way to pass resource
 // to C API.
-public fun ImmutableBinaryBlob.asCPointer(offset: Int) = interpretCPointer<ByteVar>(asCPointerImpl(offset))
+public fun ImmutableBinaryBlob.asCPointer(offset: Int = 0) =
+        interpretCPointer<ByteVar>(asCPointerImpl(offset))!!
 @SymbolName("Kotlin_ImmutableBinaryBlob_asCPointerImpl")
 private external fun ImmutableBinaryBlob.asCPointerImpl(offset: Int): kotlin.native.internal.NativePtr