DEBUG: First print the Prolog stack "safe" on a crash.

This guarantees that the stack is printed, including details on
the clauses involved and progress in each clause.   Next, the
stacks are printed with arguments.   This however may crash if
the crash is caused by corrupt data on the stacks.

src/pl-init.c

@@ -1825,8 +1825,12 @@ printCrashContext(const char *btname)
 	print_backtrace_named("GC");
       }
     }
-    Sdprintf("\n\nPROLOG STACK:\n");
-    PL_backtrace(10, btflags);
+    Sdprintf("\n\nPROLOG STACK (without arguments):\n");
+    PL_backtrace(10, PL_BT_SAFE);
+    if ( !(btflags&PL_BT_SAFE) )
+    { Sdprintf("\n\nPROLOG STACK (with arguments; may crash if data is corrupted):\n");
+      PL_backtrace(10, btflags);
+    }
   }
 
   running = false;