fixed compiler warnings

...fle.r.engine/src/com/oracle/truffle/r/engine/REngine.java

.../com/oracle/truffle/r/ffi/impl/llvm/TruffleLLVM_Call.java

...r/ffi/impl/llvm/upcalls/BytesToNativeCharArrayCallMR.java

...ruffle/r/ffi/impl/nfi/TruffleNFI_DownCallNodeFactory.java

...fle/r/library/fastrGrid/grDevices/InitWindowedDevice.java

...cle/truffle/r/nodes/builtin/base/ConnectionFunctions.java

...in/src/com/oracle/truffle/r/nodes/builtin/base/Names.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/Unlist.java

...e/truffle/r/nodes/builtin/base/foreign/ReadTableHead.java

...om/oracle/truffle/r/nodes/builtin/fastr/FastRContext.java

...om/oracle/truffle/r/nodes/builtin/fastr/FastRInterop.java

...om/oracle/truffle/r/nodes/castsTests/CastBuilderTest.java

...acle/truffle/r/nodes/access/vector/ExtractVectorNode.java

...acle/truffle/r/nodes/access/vector/ReplaceVectorNode.java

.../src/com/oracle/truffle/r/nodes/unary/PrecedenceNode.java

...me/src/com/oracle/truffle/r/runtime/context/RContext.java

...e/r/runtime/instrument/memprof/MemAllocProfilerPaths.java

.../com/oracle/truffle/r/runtime/interop/ForeignArray2R.java

.../com/oracle/truffle/r/test/library/fastr/TestInterop.java