Fix crash on SDP results

Consider folowing scenario:
1. Trigger SDP
2. Remove device (i.e. due to unbonding)
3. Trigger GATT
4. SDP results arrive -> Crash

With this patch, we would ensure that bta_dm_disc actually awaits SDP
results before parsing them, which could currently cause crash.

Test: mma -j32;
Flag: exempt, trivial
Bug: 354716690
Bug: 349228037
Change-Id: I1ef13f12137a733b9005515d1b23eeaebc1c8119