Fix R memory leak on exception

There is a possible memory leak in case the SWIG_exception_fail macro
is called. The problem is related to its definition that call the
function Rf_warning. This function (as well as Rf_error) involves
a longjmp over C++ destructors on the stack. Thus, all the objects
allocated on the heap are not freed.

Closes #914

Examples/test-suite/r/r_memory_leak_runme.R

@@ -0,0 +1,26 @@
+clargs <- commandArgs(trailing=TRUE)
+source(file.path(clargs[1], "unittest.R"))
+
+dyn.load(paste("r_memory_leak", .Platform$dynlib.ext, sep=""))
+source("r_memory_leak.R")
+cacheMetaData(1)
+
+a <- Foo();
+unittest(Foo_get_count(), 1);
+b <- Foo();
+unittest(Foo_get_count(), 2);
+
+# Normal behaviour
+invisible(trigger_internal_swig_exception("no problem", a));
+unittest(Foo_get_count(), 2);
+# SWIG exception introduced
+result <- tryCatch({
+  trigger_internal_swig_exception("null", b);
+}, warning = function(w) {
+  # print("        Hum... We received a warning, but this should be an error");
+  unittest(1,0);
+}, error = function(e) {
+  # print("        Gotcha!");
+  unittest(1,1);
+})
+unittest(Foo_get_count(), 2);