fix: missing WMALLOC NULL checks in sftpclient rm/rename

[MarkAtwood]

Bug

In the sftp client example's doCmds(), the rm and rename handlers call
WMALLOC and then dereference the result without a NULL check:

• rm: f = WMALLOC(...) then f[0] = '\0' (no check).
• rename: the first buffer f is checked, but the second, fTo = WMALLOC(...),
  is dereferenced (fTo[0] = '\0') without one.

The allocations use the system heap (NULL ctx), so on allocation failure the
result is a real NULL and the write is a NULL dereference / crash. Sibling
handlers (e.g. mkdir, and rename's first buffer) already guard.

Fix

Add the same if (... == NULL) guard the sibling handlers use. For rename's
second allocation, free the already-allocated f before returning.

Verification

Built --enable-all against wolfSSL master; compiles clean.

Reported by static analysis (Fenrir finding F-6973).

[Copilot]

Pull request overview

Fixes a NULL-dereference crash in the examples/sftpclient command handler by adding missing WMALLOC() failure checks in the rm and rename paths, consistent with other command handlers in doCmds().

Changes:

• Add WMALLOC() NULL check for the rm handler before writing to the allocated buffer.
• Add WMALLOC() NULL check for the rename destination buffer and free the already-allocated source buffer on failure.

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