fix SIGSEGV on apache 2.2

Here is SIGSEGV trace which gdb cachted

```
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffe47f3700 (LWP 4652)]
0x00007ffff6500451 in __strlen_sse2 () from /lib64/libc.so.6
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.132.el6.x86_64 keyutils-libs-1.4-4.el6.x86_64 krb5-libs-1.10.3-10.el6_4.6.x86_64 libcom_err-1.41.12-18.el6.x86_64 li
bgcc-4.4.7-4.el6.x86_64 libselinux-2.0.94-5.3.el6_4.1.x86_64 nss-softokn-freebl-3.14.3-9.el6.x86_64 openssl-1.0.1e-16.el6_5.14.x86_64 zlib-1.2.3-29.el6.x86_64

(gdb) up
#1  0x0000000000465e84 in config_log_transaction (r=0x7fffd8002970, cls=0x7405b8, default_format=0x7433c0) at mod_log_config.c:1023
1023            len += strl[i] = strlen(strs[i]);

(gdb) bt
#0  0x00007ffff6500451 in __strlen_sse2 () from /lib64/libc.so.6
#1  0x0000000000465e84 in config_log_transaction (r=0x7fffd8002970, cls=0x7405b8, default_format=0x7433c0) at mod_log_config.c:1023
#2  0x0000000000465fae in multi_log_transaction (r=0x7fffd8002970) at mod_log_config.c:1050
#3  0x000000000043a7f7 in ap_run_log_transaction (r=0x7fffd8002970) at protocol.c:1743
#4  0x00000000004aad4b in ap_process_request (r=0x7fffd8002970) at http_request.c:308
#5  0x00000000004a7baf in ap_process_http_connection (c=0x7fffec027488) at http_core.c:190
#6  0x00000000004532b9 in ap_run_process_connection (c=0x7fffec027488) at connection.c:43
#7  0x0000000000453752 in ap_process_connection (c=0x7fffec027488, csd=0x7fffec027270) at connection.c:190
#8  0x00000000004d261f in process_socket (p=0x7fffec0271f8, sock=0x7fffec027270, my_child_num=0, my_thread_num=17, bucket_alloc=0x7fffd80008e8) at worker.c:545
#9  0x00000000004d2f6e in worker_thread (thd=0x745d50, dummy=0x7fffec0008c0) at worker.c:895
#10 0x00007ffff6e9e8e3 in dummy_worker (opaque=0x745d50) at threadproc/unix/thread.c:142
#11 0x00007ffff681a9d1 in start_thread () from /lib64/libpthread.so.0
#12 0x00007ffff6567b6d in clone () from /lib64/libc.so.6
```

----

apr_psprintf returns a invalid pointer on apache 2.2 if apr_strings.h is missing.  ( I don't know why this happens :ghost: