Avoid segfaults if properties are not initialized

Null or constant dereferencing occurs if properties are not initialized.
On Android devices it shouldn't happen but can be faced if testing bionic
libc.so on Linux host.

Change-Id: I8f047cbe17d0e7bcde40ace000a8aa53789c16cb
Signed-off-by: Pavel Chupin <pavel.v.chupin@intel.com>

libc/bionic/system_properties.c

@@ -325,6 +325,8 @@ static void *to_prop_obj(prop_off_t off)
 {
     if (off > pa_data_size)
         return NULL;
+    if (!__system_property_area__)
+        return NULL;
 
     return __system_property_area__->data + off;
 }
@@ -384,6 +386,8 @@ static const prop_info *find_property(prop_bt *trie, const char *name,
 {
     const char *remaining_name = name;
 
+    if (!trie) return NULL;
+
     while (true) {
         char *sep = strchr(remaining_name, '.');
         bool want_subtree = (sep != NULL);