[Snippets] Fix invalid iterator and null pointer dereference Coverity remarks (#30437)

### Details:
Fix Coverity remarks:
 - 2360985, 2360976, 2360969: Using invalid iterator
 - 2356388: Dereference null return value

### Tickets:
 - 167167

Author: aobolensk

src/common/snippets/src/lowered/pass/mark_invariant_shape_path.cpp

@@ -46,7 +46,7 @@ static bool is_affecting_op(const ExpressionPtr& expr) {
 size_t MarkInvariantShapePath::getInvariantPortShapePath(const ExpressionPort& port) {
     auto& rt = get_rt_info(port);
     const auto rinfo = rt.find("InvariantShapePath");
-    assert(rinfo != rt.end() && "Invariant path for this expression port has not been marked!");
+    OPENVINO_ASSERT(rinfo != rt.end(), "Invariant path for this expression port has not been marked!");
     return rinfo->second.as<size_t>();
 }
 

src/common/snippets/src/op/memory_access.cpp

@@ -105,7 +105,7 @@ void MemoryAccess::set_output_port_descriptor(const PortDescriptor& desc, const
 
 const MemoryAccess::PortDescriptor& MemoryAccess::get_input_port_descriptor(const size_t i) const {
     const auto it = m_input_ports.find(i);
-    assert(it != m_input_ports.end() && "Index of input port descriptor should be less than count of input ports");
+    OPENVINO_ASSERT(it != m_input_ports.end(), "Index of input port descriptor should be less than count of input ports");
     return it->second;
 }
 
@@ -115,7 +115,7 @@ MemoryAccess::PortDescriptor& MemoryAccess::get_input_port_descriptor(const size
 
 const MemoryAccess::PortDescriptor& MemoryAccess::get_output_port_descriptor(const size_t i) const {
     const auto it = m_output_ports.find(i);
-    assert(it != m_output_ports.end() && "Index of output port descriptor should be less than count of output ports");
+    OPENVINO_ASSERT(it != m_output_ports.end(), "Index of output port descriptor should be less than count of output ports");
     return it->second;
 }
 

src/common/snippets/src/pass/transform_convert.cpp

@@ -24,6 +24,7 @@ ov::snippets::pass::TransformConvertToConvertTruncation::TransformConvertToConve
             OV_ITT_SCOPED_TASK(ov::pass::itt::domains::SnippetsTransform, "Snippets::op::TransformConvertToConvertTruncation")
             const auto root = m.get_match_root();
             const auto convert = ov::as_type_ptr<ov::opset1::Convert>(root);
+            OPENVINO_ASSERT(convert, "Convert op is invalid");
             auto convert_truncation = std::make_shared<op::ConvertTruncation>(convert->get_input_source_output(0),
                                                                               convert->get_destination_type());
             convert_truncation->set_friendly_name(convert->get_friendly_name());