Don't panic when fourth library searcher does not exists.

When disabling C modules, we remove the last two searchers (C & C all-in-one).
In Pluto the C searches may not exist by design, in this case check that 4th searcher is present before removing it.
Closes #530

Author: khvzak

src/state.rs

@@ -2062,7 +2062,6 @@ impl Lua {
         WeakLua(XRc::downgrade(&self.raw))
     }
 
-    // Luau version located in `luau/mod.rs`
     #[cfg(not(feature = "luau"))]
     fn disable_c_modules(&self) -> Result<()> {
         let package: Table = self.globals().get("package")?;
@@ -2085,7 +2084,9 @@ impl Lua {
 
         // The third and fourth searchers looks for a loader as a C library
         searchers.raw_set(3, loader)?;
-        searchers.raw_remove(4)?;
+        if searchers.raw_len() >= 4 {
+            searchers.raw_remove(4)?;
+        }
 
         Ok(())
     }

src/state/raw.rs

@@ -296,7 +296,7 @@ impl RawLua {
         if is_safe {
             let curr_libs = (*self.extra.get()).libs;
             if (curr_libs ^ (curr_libs | libs)).contains(StdLib::PACKAGE) {
-                mlua_expect!(self.lua().disable_c_modules(), "Error during disabling C modules");
+                mlua_expect!(self.lua().disable_c_modules(), "Error disabling C modules");
             }
         }
         #[cfg(feature = "luau")]