PPC: fix print-code issue with constant pool

R=bjaideep@ca.ibm.com, joransiu@ca.ibm.com

Bug: 
Change-Id: I78ec5f187559901dd4703d0964cd0c05726cd2b9
Reviewed-on: https://chromium-review.googlesource.com/553057Reviewed-by: Jaideep Bajwa <bjaideep@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#46299}

src/ppc/assembler-ppc.cc

@@ -154,7 +154,7 @@ bool RelocInfo::IsCodedSpecially() {
 
 
 bool RelocInfo::IsInConstantPool() {
-  if (FLAG_enable_embedded_constant_pool) {
+  if (FLAG_enable_embedded_constant_pool && host_ != NULL) {
     Address constant_pool = host_->constant_pool();
     return (constant_pool && Assembler::IsConstantPoolLoadStart(pc_));
   }