fix dbg_info loader treatment of nil keys, avoid bad usage of blank '.rdi' files; os_lnx_* -> lnx_*

src/raddbg/raddbg_main.c

@@ -10,13 +10,14 @@
 // [ ] value coloring view in watch window, so you can quickly scroll & see values outside of a threshold
 // [x] step out of scopes / loops
 //
-// [ ] codebase-internal barrier impl (win7/linux support)
 // [ ] asan pass
 // [ ] external window focusing bugs
-// [ ] PDB -> RDI conversion memory usage
+// [x] codebase-internal barrier impl (win7/linux support)
 // [x] policy for closing debug info which is no longer relevant?
 // [x] killing/restarting thread performance (#780)
 //
+// [ ] PDB -> RDI conversion memory usage
+//
 // [ ] project/user file improvements
 //  [ ] "default" -> "untitled"
 //  [ ] should clear default project data every time the program starts