gallivm: Prevent double delete on LLVM 3.6

std::unique_ptr takes ownership of MM, and a double delete could ensure
in case of an error,  as pointed out by Chris Vine in
https://bugs.freedesktop.org/show_bug.cgi?id=89387

Reviewed-by: Chris Vine <chris@cvine.freeserve.co.uk>

src/gallium/auxiliary/gallivm/lp_bld_misc.cpp

@@ -502,6 +502,7 @@ lp_build_create_jit_compiler_for_module(LLVMExecutionEngineRef *OutJIT,
 
 #if HAVE_LLVM >= 0x0306
        builder.setMCJITMemoryManager(std::unique_ptr<RTDyldMemoryManager>(MM));
+       MM = NULL; // onwership taken by std::unique_ptr
 #else
        builder.setMCJITMemoryManager(MM);
 #endif