[lua] Run destructors of local C++ objects on SWIG_fail

Arrange that destructors of local C++ objects in the wrapper function
get run on SWIG_fail (which calls lua_error() which calls longjmp()).

We achieve this by putting almost everything in the function in its
own block, and end that right before lua_error() at which point those
destructors will get called.

Examples/test-suite/lua/exception_memory_leak_runme.lua

@@ -0,0 +1,29 @@
+require("import")	-- the import fn
+import("exception_memory_leak")	-- import code
+eml=exception_memory_leak --alias
+
+-- catch "undefined" global variables
+local env = _ENV -- Lua 5.2
+if not env then env = getfenv () end -- Lua 5.1
+setmetatable(env, {__index=function (t,i) error("undefined global variable `"..i.."'",2) end})
+
+a = eml.Foo()
+assert(eml.Foo_get_count() == 1)
+b = eml.Foo()
+assert(eml.Foo_get_count() == 2)
+
+-- Normal behaviour
+eml.trigger_internal_swig_exception("no problem", a)
+assert(eml.Foo_get_count() == 2)
+assert(eml.Foo_get_freearg_count() == 1)
+
+-- SWIG exception triggered and handled (return new object case)
+ok,ex=pcall(eml.trigger_internal_swig_exception, "null", b)
+assert(ok==false)
+assert(eml.Foo_get_count() == 2)
+assert(eml.Foo_get_freearg_count() == 2)
+
+-- SWIG exception triggered and handled (return by value case).
+ok,ex=pcall(eml.trigger_internal_swig_exception, "null")
+assert(ok==false)
+assert(eml.Foo_get_count() == 2)