callstack: Fix list iteration in callstack_collector::merge()

Call to erase() invalidates iterators so switch from range-based for
loop to using iterators manually.

This fixes a bug that resulted in JVM crashing on SMP when "perf
callstack" was run:

  #
  # A fatal error has been detected by the Java Runtime Environment:
  #
  #  SIGSEGV (0xb) at pc=0x0000000000328a44, pid=0, tid=18446673706080178176
  #
  # JRE version: 7.0_19
  # Java VM: OpenJDK 64-Bit Server VM (23.7-b01 mixed mode linux-amd64 compressed oops)
  # Problematic frame:
  # C  0x0000000000328a44
  #
  # Core dump written. Default location: //core or core.0
  #
  # An error report file with more information is saved as:
  # /tmp/jvm-0/hs_error.log
  #
  # If you would like to submit a bug report, please include
  # instructions on how to reproduce the bug and visit:
  #   http://icedtea.classpath.org/bugzilla
  #
  Aborted

core/callstack.cc

@@ -59,12 +59,15 @@ void callstack_collector::merge()
             continue;
         }
         auto& table = **_table.for_cpu(c);
-        for (auto& tr : table) {
+        auto it = table.begin();
+        while (it != table.end()) {
+            trace& tr = *it;
             auto i = table0.find(tr);
             if (i != table0.end()) {
                 i->hits += tr.hits;
+                ++it;
             } else {
-                table.erase(table.iterator_to(tr));
+                table.erase(it++);
                 table0.insert(tr);
             }
         }