Source is

395:  => macroblock_params1_and_mv_t *pMbMvs         = EncReconCtlbufM->pMbMvs[0];
396:  memset(pMbParams, 0, sizeof(macroblock_storage_t));

GDB output at break at line 395 is

Program received signal SIGTRAP, Trace/breakpoint trap.
0x04135454 in MediaH264EncodeCoreImp::EncodeMb (this=0x4364920, ctx=0x4364bcc, mb=0x5c149120) at h264EncMb.cpp:395
heuristic_proc_start::pc=2aed72c0
warning: GDB can't find the start of the function at 0x2aed72c0.
heuristic_proc_start::pc=2aed72c0
warning: GDB can't find the start of the function at 0x2aed72c0.
heuristic_proc_start::pc=2aed72c0
<...>

GDB hangs here and prints warning in a endless loop :(