Dwarf: Return that function isn't found if function list is empty.

Author: southpolenator

Source/CsDebugScript.DwarfSymbolProvider/DwarfSymbolProviderModule.cs

@@ -2134,6 +2134,11 @@ private DwarfSymbol FindFunction(ulong address, out ulong displacement)
             {
                 index = functionsCache.Count - 1;
             }
+            if (index < 0)
+            {
+                displacement = address;
+                return null;
+            }
             if (functionAddressesCache[index] > address && index > 0)
             {
                 index--;