"thread until": make sure the current function is valid before asking it questions

[jingham]

Summary

Thread until was assuming that if you have debug information, you will have a function. Apparently some hand-crafted cross-library stubs can have debug information but no functions, and that was causing lldb to crash. This patch just adds a check for the function as well, and we error out if we don't have a function. The patch is sufficiently obvious I don't think it warrants the effort to make one of these odd constructs.

[JDevlieghere]

Makes sense. LGTM.

lldb/source/Commands/CommandObjectThread.cpp

if (process == nullptr) {
                                        m_options.m_thread_idx);
           return false;
         }
 
         LineEntry function_start;
         uint32_t index_ptr = 0, end_ptr;
         std::vector<addr_t> address_list;
 
-        // Find the beginning & end index of the        
+        // Find the beginning & end index of the function, but first make
+        // sure it is valid:
+        if (!sc.function) {
+          result.AppendErrorWithFormat("Have debug information but no "
+                                       "function info - can't get until range.");
+          return false;
+        }
+
         AddressRange fun_addr_range = sc.function->GetAddressRange();
         Address fun_start_addr = fun_addr_range.GetBaseAddress();
         line_table->FindLineEntryByAddress(fun_start_addr, function_start,
                                            &index_ptr);
 
         Address fun_end_addr(fun_start_addr.GetSection(),
                              fun_start_addr.GetOffset() +
                                  fun_addr_range.GetByteSize());