modules/core/src/ocl.cpp: Fix dangling pointer

Detected by clang trunk:
```
opencv/modules/core/src/ocl.cpp:4337:37: warning: object backing the pointer will be destroyed at the end of the full-expression [-Wdangling]
        CV_OCL_CHECK_RESULT(retval, cv::format("clCreateBuffer(capacity=%lld) => %p", (long long int)entry.capacity_, (void*)entry.clBuffer_).c_str());
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
opencv/modules/core/src/ocl.cpp:193:42: note: expanded from macro 'CV_OCL_CHECK_RESULT'
            if (0) { const char* msg_ = (msg); CV_UNUSED(msg_); /* ensure const char* type (cv::String without c_str()) */ } \
```
because `cv::format` yields a temporary std::string, and thus `msg_` points to a destroyed buffer.

modules/core/src/ocl.cpp

@@ -190,7 +190,7 @@ void traceOpenCLCheck(cl_int status, const char* message)
         CV_OCL_TRACE_CHECK_RESULT(check_result, msg); \
         if (check_result != CL_SUCCESS) \
         { \
-            if (0) { const char* msg_ = (msg); CV_UNUSED(msg_); /* ensure const char* type (cv::String without c_str()) */ } \
+            static_assert(std::is_convertible<decltype(msg), const char*>::value, "msg of CV_OCL_CHECK_RESULT must be const char*"); \
             cv::String error_msg = CV_OCL_API_ERROR_MSG(check_result, msg); \
             CV_Error(Error::OpenCLApiCallError, error_msg); \
         } \
@@ -210,7 +210,7 @@ void traceOpenCLCheck(cl_int status, const char* message)
         CV_OCL_TRACE_CHECK_RESULT(check_result, msg); \
         if (check_result != CL_SUCCESS && isRaiseError()) \
         { \
-            if (0) { const char* msg_ = (msg); CV_UNUSED(msg_); /* ensure const char* type (cv::String without c_str()) */ } \
+            static_assert(std::is_convertible<decltype(msg), const char*>::value, "msg of CV_OCL_DBG_CHECK_RESULT must be const char*"); \
             cv::String error_msg = CV_OCL_API_ERROR_MSG(check_result, msg); \
             CV_Error(Error::OpenCLApiCallError, error_msg); \
         } \