Fix crash in Rust expression parser

A user found that an array expression with just a single value (like
"[23]") caused the Rust expression parser to crash.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30410

gdb/rust-parse.c

@@ -1190,7 +1190,7 @@ rust_parser::parse_array ()
       result = make_operation<rust_array_operation> (std::move (expr),
 						     std::move (rhs));
     }
-  else if (current_token == ',')
+  else if (current_token == ',' || current_token == ']')
     {
       std::vector<operation_up> ops;
       ops.push_back (std::move (expr));
@@ -1205,7 +1205,7 @@ rust_parser::parse_array ()
       int len = ops.size () - 1;
       result = make_operation<array_operation> (0, len, std::move (ops));
     }
-  else if (current_token != ']')
+  else
     error (_("',', ';', or ']' expected"));
 
   require (']');

gdb/testsuite/gdb.rust/expr.exp

@@ -113,6 +113,7 @@ gdb_test "ptype \[1,2,3,4\]" "type = \\\[i32; 4\\\]"
 gdb_test "print \[mut 1,2,3,4\]" " = \\\[1, 2, 3, 4\\\]"
 gdb_test "print \[1,2 3" "',' or ']' expected"
 gdb_test "print \[1 2" "',', ';', or ']' expected"
+gdb_test "print \[23\]" " = \\\[23\\\]"
 
 gdb_test "print b\"hi rust\"" " = b\"hi rust\""
 # This isn't rusty syntax yet, but that's another bug -- this is just