8010144: [parfait] Null pointer deference in hotspot/src/os_cpu/linux_x86/vm/os_linux_x86.cpp

Summary: add null check to signal handler
Reviewed-by: dcubed

src/os_cpu/linux_x86/vm/os_linux_x86.cpp

@@ -340,7 +340,7 @@ JVM_handle_linux_signal(int sig,
         // here if the underlying file has been truncated.
         // Do not crash the VM in such a case.
         CodeBlob* cb = CodeCache::find_blob_unsafe(pc);
-        nmethod* nm = cb->is_nmethod() ? (nmethod*)cb : NULL;
+        nmethod* nm = (cb != NULL && cb->is_nmethod()) ? (nmethod*)cb : NULL;
         if (nm != NULL && nm->has_unsafe_access()) {
           stub = StubRoutines::handler_for_unsafe_access();
         }