fix out-of-bounds X underflow in SBR low-power QMF assembly

[netliomax25-code]

1. In SBR_LOW_POWER builds, sbr_process_channel() (libfaad/sbr_dec.c) finishes each output time slot of the QMF matrix X with an overlap add:

   QMF_RE(X[l][kx_band - 1 + bsco_band]) +=
       QMF_RE(sbr->Xsbr[ch][l + sbr->tHFAdj][kx_band - 1 + bsco_band]);

2. For the leading time slots (l < sbr->t_E[ch][0]) the band parameters are taken from the previous frame: kx_band = sbr->kx_prev and bsco_band = sbr->bsco_prev.

3. On the very first decoded SBR frame both are still 0: sbr_info is zero-initialised, and sbr_save_prev_data() — the only writer of kx_prev/bsco_prev — runs after the frame is processed.

4. A first frame whose grid is VARFIX or VARVAR with bs_abs_bord >= 1 sets abs_bord_lead > 0, so t_E[ch][0] = rate * abs_bord_lead > 0. The l = 0 slot then takes the kx_prev path, and kx_band - 1 + bsco_band evaluates to -1, so the statement reads and writes X[0][-1] — one real_t before the stack array X[MAX_NTSRHFG][64]. AddressSanitizer reports a stack-buffer-underflow at sbr_dec.c:447.

5. The current-frame kx is always >= 5 (the qmf_start_channel() start tables and derived_frequency_table()), so the only way kx_band + bsco_band can be 0 is the first-frame kx_prev == 0 / bsco_prev == 0 case. This is a different site from the hf_assembly() upper-bound (m + kx + 1 == 64) overflow.

6. Fix: guard the overlap add with if (kx_band + bsco_band > 0), skipping it when there is no band below 0 to add. Normal frames (kx_band >= 5) are unaffected and the high-quality (non-low-power) path does not contain this statement.

7. Verified with a minimal crafted SBR payload driven through sbr_extension_data() + sbrDecodeSingleFrame() in an SBR_LOW_POWER + ASan build: stack-buffer-underflow at sbr_dec.c:447 before the change, clean afterwards (ret == 0, decode continues). The default and SBR_LOW_POWER configurations both build clean with -Wall.