Fix string index error in tab completion code, fixes #51540 (#51541)

(cherry picked from commit 6ce15fc)

stdlib/REPL/src/REPLCompletions.jl

@@ -943,7 +943,7 @@ function complete_identifiers!(suggestions::Vector{Completion}, @nospecialize(ff
     ex = nothing
     comp_keywords && append!(suggestions, complete_keyword(name))
     if dotpos > 1 && string[dotpos] == '.'
-        s = string[1:dotpos-1]
+        s = string[1:prevind(string, dotpos)]
         # First see if the whole string up to `pos` is a valid expression. If so, use it.
         ex = Meta.parse(s, raise=false, depwarn=false)
         if isexpr(ex, :incomplete)

stdlib/REPL/test/replcompletions.jl

@@ -23,6 +23,8 @@ let ex = quote
         end
         type_test = Test_x(Test_y(1))
         (::Test_y)() = "", ""
+        unicode_αβγ = Test_y(1)
+
         module CompletionFoo2
 
         end
@@ -253,6 +255,11 @@ let s = "Main.CompletionFoo.type_test.x"
     @test s[r] == "x"
 end
 
+let s = "Main.CompletionFoo.unicode_αβγ.y"
+    c, r = test_complete(s)
+    @test "yy" in c
+end
+
 let s = "Main.CompletionFoo.bar.no_val_available"
     c, r = test_complete(s)
     @test length(c)==0