AP_Filesystem: guarantee load_file() data is null-terminated

Improves safety of use and clarity of users. Termination is not
included in the reported size to avoid changing user behavior or
misrepresenting the file contents.

libraries/AP_Filesystem/AP_Filesystem_ROMFS.cpp

@@ -239,8 +239,9 @@ bool AP_Filesystem_ROMFS::set_mtime(const char *filename, const uint32_t mtime_s
 }
 
 /*
-  load a full file. Use delete to free the data
-  we override this in ROMFS to avoid taking twice the memory
+  Load a file's contents into memory. Returned object must be `delete`d to free
+  the data. The data is guaranteed to be null-terminated such that it can be
+  treated as a string. Overridden in ROMFS to avoid taking twice the memory.
 */
 FileData *AP_Filesystem_ROMFS::load_file(const char *filename)
 {
@@ -248,6 +249,7 @@ FileData *AP_Filesystem_ROMFS::load_file(const char *filename)
     if (!fd) {
         return nullptr;
     }
+    // AP_ROMFS adds the guaranteed termination so we don't have to.
     fd->data = AP_ROMFS::find_decompress(filename, fd->length);
     if (fd->data == nullptr) {
         delete fd;