fix some 'vfprintf %s NULL'

Index: src/base/lnav_log.cc
--- src/base/lnav_log.cc.orig
+++ src/base/lnav_log.cc
@@ -228,11 +228,13 @@ log_host_info()
     log_info("  jittarget=%s", jittarget);
     log_info("Environment:");
     log_info("  HOME=%s", getenv("HOME"));
-    log_info("  XDG_CONFIG_HOME=%s", getenv("XDG_CONFIG_HOME"));
+    if (getenv("XDG_CONFIG_HOME") != nullptr)
+        log_info("  XDG_CONFIG_HOME=%s", getenv("XDG_CONFIG_HOME"));
     log_info("  LANG=%s", getenv("LANG"));
     log_info("  PATH=%s", getenv("PATH"));
     log_info("  TERM=%s", getenv("TERM"));
-    log_info("  TZ=%s", getenv("TZ"));
+    if (getenv("TZ") != nullptr)
+        log_info("  TZ=%s", getenv("TZ"));
     log_info("Process:");
     log_info("  pid=%d", getpid());
     log_info("  ppid=%d", getppid());
