Fix MAP_FAILED detection in pmem_mmap (#298)

mmap(2) returns MAP_FAILED ((void*)-1) on error, not NULL. The check
'if (likely(ret))' treats MAP_FAILED as truthy, so the emergency retry
path is never reached and callers receive MAP_FAILED as a valid pointer.

Normalize pmem_mmap() to return NULL on failure (converting MAP_FAILED
to NULL in the mmap path). This confines the platform-specific error
handling to one place and eliminates the need for preprocessor checks
in pmem_mmap_safe() and psync_mmap_anon_emergency().

Fixes GH #226

Co-authored-by: Levi Neely <lkn@darkstar.example.net>

pclsync/pmem.c

@@ -6,8 +6,9 @@
 
 void *pmem_mmap(size_t size) {
 #if defined(MAP_ANONYMOUS)
-  return mmap(NULL, size, PROT_READ | PROT_WRITE,
-              MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  void *ret = mmap(NULL, size, PROT_READ | PROT_WRITE,
+                   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  return (ret == MAP_FAILED) ? NULL : ret;
 #else
   return malloc(size);
 #endif