* Prevent crashes and leaks on allocation failure

Memory allocation can fail. We need to gracefully handle this case and
prevent dereferencing null pointers.

* Make NULL checks consistent within a file

* Properly report allocation errors

* Improve cleanup in pj_gc_reader.c

* Implement pj_strdup and use instead of strdup

The function strdup is not part of ANSI C 89, but a POSIX extension.
Therefore we can not rely on it being available on all platforms.

* pj_transform() crashes on a catalog that has no matching grid
* pj_free() and pj_gc_unloadall() badly interact. No longer try to free the
  catalog object in pj_free(). That is the job of pj_gc_unloadall()
* Fix memory leaks in pj_gc_readcatalog() and pj_gc_unloadall()

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1923

Credit to OSS Fuzz.