Fix crash when using uninstantiated generic (#22379)

* Add test case

* Add in a bounds check when accessing generic types

Removes idnex out of bounds exception when comparing a generic that isn't fully instantiated

compiler/sigmatch.nim

@@ -1536,6 +1536,8 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
     if x.kind == tyGenericInvocation:
       if f[0] == x[0]:
         for i in 1..<f.len:
+          # Handle when checking against a generic that isn't fully instantiated
+          if i >= x.len: return
           let tr = typeRel(c, f[i], x[i], flags)
           if tr <= isSubtype: return
         result = isGeneric

tests/generics/tuninstantiated_failure.nim

@@ -0,0 +1,16 @@
+discard """
+cmd: "nim check $file"
+"""
+
+type
+  Test[T, K] = object
+    name: string
+  Something = Test[int]
+
+func `[]`[T, K](x: var Test[T, K], idx: int): var Test[T, K] =
+  x
+
+var b: Something
+# Should give a type-mismatch since Something isn't a valid Test
+b[0].name = "Test" #[tt.Error
+            ^  type mismatch]#