nvhost_nvdec_common: Avoid memcpy with null pointers

Avoid sending null pointer to memcpy as reported by Undefined Behavious
Sanitizer.

Co-authored-by: LC <mathew1800@gmail.com>

src/core/hle/service/nvdrv/devices/nvhost_nvdec_common.cpp

@@ -23,17 +23,22 @@ namespace {
 template <typename T>
 std::size_t SpliceVectors(const std::vector<u8>& input, std::vector<T>& dst, std::size_t count,
                           std::size_t offset) {
+    if (!dst.empty()) {
         std::memcpy(dst.data(), input.data() + offset, count * sizeof(T));
-    offset += count * sizeof(T);
-    return offset;
+    }
+    return 0;
 }
 
 // Write vectors will write data to the output buffer
 template <typename T>
 std::size_t WriteVectors(std::vector<u8>& dst, const std::vector<T>& src, std::size_t offset) {
+    if (src.empty()) {
+        return 0;
+    } else {
         std::memcpy(dst.data() + offset, src.data(), src.size() * sizeof(T));
         offset += src.size() * sizeof(T);
         return offset;
+    }
 }
 } // Anonymous namespace