free_pattern_spec(): Don't try to free an invalid pointer

free_pattern_spec(): Don't try to free an invalid pointer

... when unloading an action file with a TAG pattern while
Privoxy has been compiled without FEATURE_PCRE_HOST_PATTERNS.

   Thread 1 received signal SIGSEGV, Segmentation fault.
   0x00000008015a8bab in regfree (preg=0x800000000) at pcreposix.c:248
   248	pcreposix.c: No such file or directory.
   (gdb) where
   #0  0x00000008015a8bab in regfree (preg=0x800000000) at pcreposix.c:248
   #1  0x000000000045783a in free_pattern_spec (pattern=0x8029b9110) at urlmatch.c:1284
   #2  0x000000000040705f in unload_actions_file (file_data=0x8029b9070) at actions.c:1006
   #3  0x000000000044a146 in sweep () at loaders.c:248
   #4  0x0000000000439bfa in listen_loop () at jcc.c:6230
   #5  0x0000000000439456 in main (argc=3, argv=0x7fffffffe728) at jcc.c:5726
   (gdb) f 1
   #1  0x000000000045783a in free_pattern_spec (pattern=0x8029b9110) at urlmatch.c:1284
   1284	      regfree(pattern->pattern.tag_regex);
   (gdb) p *pattern
   $1 = {spec = 0x0, pattern = {url_spec = {dcount = 0, dbuffer = 0x0, dvec = 0x0, unanchored = 0, port_list = 0x0, preg = 0x0}, tag_regex = 0x800000000}, flags = 16}

Closes: SF patch request #147