[SYCL] Initialize uninitialized handler_impl fields (#15323)

In the same vein as #15237, this PR
fixes additional uninitialized values recently discovered by Coverity.
Similar to the resolution discussed in #15237, I have
default-initialized integer values that are defined later on to 0
instead of another more complex solution.

Additionally, since I had set `MExternalSempahore` in `handler_impl` to
`nullptr`, I added null checks where `MExternalSemaphore` is ultimately
returned to ensure `nullptr` doesn't actually get passed into the UR.
This is not necessarily necessary, but without this check Coverity would
probably generate another hit because of it.

Author: ianayl

sycl/source/detail/cg.hpp

@@ -624,6 +624,8 @@ class CGSemaphoreWait : public CG {
         MExternalSemaphore(ExternalSemaphore), MWaitValue(WaitValue) {}
 
   ur_exp_external_semaphore_handle_t getExternalSemaphore() const {
+    assert(MExternalSemaphore != nullptr &&
+           "MExternalSemaphore has not been defined yet.");
     return MExternalSemaphore;
   }
   std::optional<uint64_t> getWaitValue() const { return MWaitValue; }
@@ -643,6 +645,10 @@ class CGSemaphoreSignal : public CG {
         MExternalSemaphore(ExternalSemaphore), MSignalValue(SignalValue) {}
 
   ur_exp_external_semaphore_handle_t getExternalSemaphore() const {
+    if (MExternalSemaphore == nullptr)
+      throw exception(make_error_code(errc::runtime),
+                      "getExternalSemaphore(): MExternalSemaphore has not been "
+                      "defined yet.");
     return MExternalSemaphore;
   }
   std::optional<uint64_t> getSignalValue() const { return MSignalValue; }

sycl/source/detail/handler_impl.hpp

@@ -90,13 +90,13 @@ class handler_impl {
 
   std::shared_ptr<detail::kernel_bundle_impl> MKernelBundle;
 
-  ur_usm_advice_flags_t MAdvice;
+  ur_usm_advice_flags_t MAdvice = 0;
 
   // 2D memory operation information.
-  size_t MSrcPitch;
-  size_t MDstPitch;
-  size_t MWidth;
-  size_t MHeight;
+  size_t MSrcPitch = 0;
+  size_t MDstPitch = 0;
+  size_t MWidth = 0;
+  size_t MHeight = 0;
 
   /// Offset into a device_global for copy operations.
   size_t MOffset = 0;
@@ -134,7 +134,7 @@ class handler_impl {
   ur_rect_region_t MCopyExtent = {};
 
   // Extra information for semaphore interoperability
-  ur_exp_external_semaphore_handle_t MExternalSemaphore;
+  ur_exp_external_semaphore_handle_t MExternalSemaphore = nullptr;
   std::optional<uint64_t> MWaitValue;
   std::optional<uint64_t> MSignalValue;