Fix allocation in CROSSCOMPILING_EMULATOR evaluation

In commit fec441ec17 (Teach CROSSCOMPILING_EMULATOR to support
arguments, 2019-05-30, v3.15.0-rc1~6^2) the new member
`cmCustomCommandGenerator::EmulatorsWithArguments` was not initialized
to the proper size.  Fix this and add a test case covering the crash
that could occur with multiple commands where an emulator appears only
in a later command.

Fixes: #19500
Co-Author: Brad King <[email protected]>

Source/cmCustomCommandGenerator.cxx

@@ -25,6 +25,7 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(cmCustomCommand const& cc,
   , OldStyle(cc.GetEscapeOldStyle())
   , MakeVars(cc.GetEscapeAllowMakeVars())
   , GE(new cmGeneratorExpression(cc.GetBacktrace()))
+  , EmulatorsWithArguments(cc.GetCommandLines().size())
 {
   const cmCustomCommandLines& cmdlines = this->CC.GetCommandLines();
   for (cmCustomCommandLine const& cmdline : cmdlines) {
@@ -107,7 +108,6 @@ void cmCustomCommandGenerator::FillEmulatorsWithArguments()
         continue;
       }
 
-      this->EmulatorsWithArguments.emplace_back();
       cmSystemTools::ExpandListArgument(emulator_property,
                                         this->EmulatorsWithArguments[c]);
     }

Tests/RunCMake/CrosscompilingEmulator/AddCustomCommand.cmake

@@ -26,12 +26,14 @@ add_custom_command(OUTPUT output2
 
 # DoesNotUseEmulator: The command will fail if emulator is prepended
 add_custom_command(OUTPUT output3
+  COMMAND ${CMAKE_COMMAND} -E echo generated_exe_emulator_unexpected
   COMMAND $<TARGET_FILE:generated_exe_emulator_unexpected>
   COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/output3
   DEPENDS generated_exe_emulator_unexpected)
 
 # DoesNotUseEmulator: The command will fail if emulator is prepended
 add_custom_command(OUTPUT outputImp
+  COMMAND ${CMAKE_COMMAND} -E echo generated_exe_emulator_unexpected_imported
   COMMAND generated_exe_emulator_unexpected_imported
   COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/outputImp
   )
@@ -39,6 +41,7 @@ add_custom_command(OUTPUT outputImp
 # UsesEmulator: The command only succeeds if the emulator is prepended
 #               to the command.
 add_custom_command(OUTPUT output4
+  COMMAND ${CMAKE_COMMAND} -E echo generated_exe_emulator_expected
   COMMAND generated_exe_emulator_expected
   COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/output4
   DEPENDS generated_exe_emulator_expected)