Fixes zealous MSVC static analyzers warnings (#3938)

Other unfixed as I'm not happy with caving to false positives of every analyzers.

backends/imgui_impl_win32.cpp

@@ -193,6 +193,7 @@ static bool ImGui_ImplWin32_UpdateMouseCursor()
 static void ImGui_ImplWin32_UpdateMousePos()
 {
     ImGuiIO& io = ImGui::GetIO();
+    IM_ASSERT(g_hWnd != 0);
 
     // Set OS mouse position if requested (rarely used, only when ImGuiConfigFlags_NavEnableSetMousePos is enabled by user)
     if (io.WantSetMousePos)

imgui.cpp

@@ -1133,11 +1133,18 @@ void ImGuiIO::AddInputCharacterUTF16(ImWchar16 c)
     if (InputQueueSurrogate != 0)
     {
         if ((c & 0xFC00) != 0xDC00) // Invalid low surrogate
+        {
             InputQueueCharacters.push_back(IM_UNICODE_CODEPOINT_INVALID);
-        else if (IM_UNICODE_CODEPOINT_MAX == (0xFFFF)) // Codepoint will not fit in ImWchar (extra parenthesis around 0xFFFF somehow fixes -Wunreachable-code with Clang)
-            cp = IM_UNICODE_CODEPOINT_INVALID;
+        }
         else
+        {
+#if IM_UNICODE_CODEPOINT_MAX == 0xFFFF
+            cp = IM_UNICODE_CODEPOINT_INVALID; // Codepoint will not fit in ImWchar
+#else
             cp = (ImWchar)(((InputQueueSurrogate - 0xD800) << 10) + (c - 0xDC00) + 0x10000);
+#endif
+        }
+
         InputQueueSurrogate = 0;
     }
     InputQueueCharacters.push_back(cp);

imgui_tables.cpp

@@ -2678,6 +2678,7 @@ void ImGui::TableSortSpecsBuild(ImGuiTable* table)
     // Write output
     table->SortSpecsMulti.resize(table->SortSpecsCount <= 1 ? 0 : table->SortSpecsCount);
     ImGuiTableColumnSortSpecs* sort_specs = (table->SortSpecsCount == 0) ? NULL : (table->SortSpecsCount == 1) ? &table->SortSpecsSingle : table->SortSpecsMulti.Data;
+    if (sort_specs != NULL)
         for (int column_n = 0; column_n < table->ColumnsCount; column_n++)
         {
             ImGuiTableColumn* column = &table->Columns[column_n];