Small fix to the zeroing of the SME regs before attempting reads

When the process is not in Streaming SVE Mode, all of the SME
thread_get_state calls will fail.  Zero out the buffers that we
were going to write data into, so we don't leave old data in them
that might be sent to lldb (particularly in a GetRegisterContext
"g" packet response, where we need to read all registers to complete
the response).

Author: jasonmolenda

lldb/tools/debugserver/source/MacOSX/arm64/DNBArchImplARM64.cpp

@@ -515,6 +515,9 @@ kern_return_t DNBArchMachARM64::GetSMEState(bool force) {
   // will fail, and we may return uninitialized data in the register context.
   memset(&m_state.context.sme.svcr, 0, ARM_SME_STATE_COUNT * sizeof(uint32_t));
   memset(m_state.context.sme.za.data(), 0, m_state.context.sme.za.size());
+  if (CPUHasSME2())
+    memset(&m_state.context.sme.zt0, 0,
+           ARM_SME2_STATE_COUNT * sizeof(uint32_t));
 
   // Read the registers from our thread
   mach_msg_type_number_t count = ARM_SME_STATE_COUNT;
@@ -526,7 +529,6 @@ kern_return_t DNBArchMachARM64::GetSMEState(bool force) {
   if (kret != KERN_SUCCESS)
     return kret;
 
-  memset(m_state.context.sme.za.data(), 0, m_state.context.sme.za.size());
 
   size_t za_size = m_state.context.sme.svl_b * m_state.context.sme.svl_b;
   const size_t max_chunk_size = 4096;
@@ -553,8 +555,6 @@ kern_return_t DNBArchMachARM64::GetSMEState(bool force) {
   }
 
   if (CPUHasSME2()) {
-    memset(&m_state.context.sme.zt0, 0,
-           ARM_SME2_STATE_COUNT * sizeof(uint32_t));
     count = ARM_SME2_STATE_COUNT;
     kret = thread_get_state(m_thread->MachPortNumber(), ARM_SME2_STATE,
                             (thread_state_t)&m_state.context.sme.zt0, &count);