fix coredump when fopen failed

src/share/ob_tenant_mgr.cpp

@@ -26,13 +26,12 @@
 
 int64_t get_virtual_memory_used()
 {
-  constexpr int BUFFER_SIZE = 128;
-  char filename[BUFFER_SIZE];
   int64_t page_cnt = 0;
-  snprintf(filename, BUFFER_SIZE, "/proc/%d/statm", getpid());
-  FILE* statm = fopen(filename, "r");
-  fscanf(statm, "%ld", &page_cnt);
-  fclose(statm);
+  FILE* statm = fopen("/proc/self/statm", "r");
+  if (OB_NOT_NULL(statm)) {
+    fscanf(statm, "%ld", &page_cnt);
+    fclose(statm);
+  }
   return page_cnt * sysconf(_SC_PAGESIZE);
 }