fix a bug , we inc number of inst without checking if inst is null (gpgpu-sim#114)

Author: LAhmos

src/gpgpu-sim/shader.cc

@@ -891,9 +891,9 @@ void shader_core_ctx::decode() {
     // decode 1 or 2 instructions and place them into ibuffer
     address_type pc = m_inst_fetch_buffer.m_pc;
     const warp_inst_t *pI1 = get_next_inst(m_inst_fetch_buffer.m_warp_id, pc);
-    m_warp[m_inst_fetch_buffer.m_warp_id]->ibuffer_fill(0, pI1);
-    m_warp[m_inst_fetch_buffer.m_warp_id]->inc_inst_in_pipeline();
-    if (pI1) {
+      if (pI1) {
+      m_warp[m_inst_fetch_buffer.m_warp_id]->ibuffer_fill(0, pI1);
+      m_warp[m_inst_fetch_buffer.m_warp_id]->inc_inst_in_pipeline();
       m_stats->m_num_decoded_insn[m_sid]++;
       if ((pI1->oprnd_type == INT_OP) ||
           (pI1->oprnd_type == UN_OP)) {  // these counters get added up in mcPat