[runtime] Fixed marshalling bug in `mono_marshal_get_native_func_wrap…

…per`.

It was caused by a mismatch of the `sig` and `csig` variables when looking the wrapper in the marshalling cache.

This was manifesting as the following crash in all Monodroid programs:

```
#0  mono_type_hash () at /Users/joao/Dev/droid/mono/mono/metadata/metadata.c:1390
#1  0x7512edf4 in mono_signature_hash () at /Users/joao/Dev/droid/mono/mono/metadata/metadata.c:4957
#2  0x75115d18 in signature_pointer_pair_hash () from /Users/joao/Dev/droid/monodroid/tests/runtime/gdb-symbols/libmonosgen-2.0.so
#3  0x751dca10 in rehash () from /Users/joao/Dev/droid/monodroid/tests/runtime/gdb-symbols/libmonosgen-2.0.so
#4  0x751dcbe0 in monoeg_g_hash_table_insert_replace () from /Users/joao/Dev/droid/monodroid/tests/runtime/gdb-symbols/libmonosgen-2.0.so
#5  0x75119570 in mono_mb_create_and_cache_full () from /Users/joao/Dev/droid/monodroid/tests/runtime/gdb-symbols/libmonosgen-2.0.so
#6  0x75127390 in mono_marshal_get_native_func_wrapper () from /Users/joao/Dev/droid/monodroid/tests/runtime/gdb-symbols/libmonosgen-2.0.so
#7  0x75127780 in mono_ftnptr_to_delegate () from /Users/joao/Dev/droid/monodroid/tests/runtime/gdb-symbols/libmonosgen-2.0.so
```

Originally introduced in #1540.

mono/metadata/marshal.c

@@ -7375,7 +7375,10 @@ mono_marshal_get_native_func_wrapper (MonoImage *image, MonoMethodSignature *sig
 	gboolean found;
 	char *name;
 
-	key.sig = sig;
+	csig = signature_dup (image, sig);
+	csig->pinvoke = 0;
+
+	key.sig = csig;
 	key.pointer = func;
 
 	cache = get_cache (&image->native_func_wrapper_cache, signature_pointer_pair_hash, signature_pointer_pair_equal);
@@ -7390,9 +7393,6 @@ mono_marshal_get_native_func_wrapper (MonoImage *image, MonoMethodSignature *sig
 	mono_marshal_emit_native_wrapper (image, mb, sig, piinfo, mspecs, func, FALSE, TRUE, FALSE);
 #endif
 
-	csig = signature_dup (image, sig);
-	csig->pinvoke = 0;
-
 	new_key = g_new (SignaturePointerPair,1);
 	*new_key = key;
 	res = mono_mb_create_and_cache_full (cache, new_key, mb, csig, csig->param_count + 16, NULL, &found);