Prevent segfault if file cannot be opened when --ll_prof is used.

src/platform-openbsd.cc

Index: src/platform-openbsd.cc
diff --git a/src/platform-openbsd.cc b/src/platform-openbsd.cc
index 380c15f21a3abd56ffde7d81c31a7e3c6b9532bc..0a7cc80f3db1538a1b65be5b4ff21a549c3d16ad 100644
--- a/src/platform-openbsd.cc
+++ b/src/platform-openbsd.cc
@@ -344,6 +344,10 @@ void OS::SignalCodeMovingGC() {
   // kernel log.
   int size = sysconf(_SC_PAGESIZE);
   FILE* f = fopen(FLAG_gc_fake_mmap, "w+");
+  if (f == NULL) {
+    OS::PrintError("Failed to open %s\n", FLAG_gc_fake_mmap);
+    OS::Abort();
+  }
   void* addr = mmap(NULL, size, PROT_READ | PROT_EXEC, MAP_PRIVATE,
                     fileno(f), 0);
   ASSERT(addr != MAP_FAILED);