check realloc result before zeroing grown bytes in loader_realloc

[aizu-m]

Was tracing the generic-list growth paths and stopped on loader_realloc.

1. on the default path we store realloc(pMemory, size) in pNewMem, then zero the grown tail with memset((uint8_t *)pNewMem + orig_size, 0, size - orig_size) whenever size > orig_size.
2. realloc hands back NULL on failure and leaves the original block untouched, so under OOM pNewMem is NULL and that memset writes through a null pointer at offset orig_size before we ever return.

The callers all check the returned pointer for NULL, so only the zero-fill inside loader_realloc touches it early. Guarded the memset on pNewMem so the tail is only cleared when realloc actually returned the resized block. Behaviour is unchanged on success.

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