[ruby/yarp] Fix string delimiter with "@" confused for embedded

variable
(https://github.com/ruby/yarp/pull/1521)

https://github.com/ruby/yarp/commit/3ec0699622

test/yarp/errors_test.rb

@@ -152,6 +152,13 @@ module YARP
       ]
     end
 
+    def test_incomplete_instance_var_string
+      assert_errors expression('%@#@@#'), '%@#@@#', [
+        ["Incomplete instance variable", 4..5],
+        ["Expected a newline or semicolon after the statement", 4..4]
+      ]
+    end
+
     def test_unterminated_s_symbol
       assert_errors expression("%s[abc"), "%s[abc", [
         ["Expected a closing delimiter for the dynamic symbol", 3..3]

test/yarp/fixtures/strings.txt

@@ -99,3 +99,7 @@
 ?a "a"
 
 %Q{abc}
+
+%^#$^#
+
+%@#@#

test/yarp/snapshots/strings.txt

@@ -1,8 +1,8 @@
-@ ProgramNode (location: (0...506))
+@ ProgramNode (location: (0...520))
 ├── locals: []
 └── statements:
-    @ StatementsNode (location: (0...506))
-    └── body: (length: 48)
+    @ StatementsNode (location: (0...520))
+    └── body: (length: 50)
         ├── @ StringNode (location: (0...6))
         │   ├── flags: ∅
         │   ├── opening_loc: (0...2) = "%%"
@@ -496,9 +496,21 @@
         │       ├── content_loc: (495...496) = "a"
         │       ├── closing_loc: (496...497) = "\""
         │       └── unescaped: "a"
-        └── @ StringNode (location: (499...506))
+        ├── @ StringNode (location: (499...506))
+        │   ├── flags: ∅
+        │   ├── opening_loc: (499...502) = "%Q{"
+        │   ├── content_loc: (502...505) = "abc"
+        │   ├── closing_loc: (505...506) = "}"
+        │   └── unescaped: "abc"
+        ├── @ StringNode (location: (508...513))
+        │   ├── flags: ∅
+        │   ├── opening_loc: (508...510) = "%^"
+        │   ├── content_loc: (510...512) = "\#$"
+        │   ├── closing_loc: (512...513) = "^"
+        │   └── unescaped: "\#$"
+        └── @ StringNode (location: (516...520))
             ├── flags: ∅
-            ├── opening_loc: (499...502) = "%Q{"
-            ├── content_loc: (502...505) = "abc"
-            ├── closing_loc: (505...506) = "}"
-            └── unescaped: "abc"
+            ├── opening_loc: (516...518) = "%@"
+            ├── content_loc: (518...519) = "#"
+            ├── closing_loc: (519...520) = "@"
+            └── unescaped: "#"

yarp/yarp.c

@@ -5801,7 +5801,7 @@ lex_interpolation(yp_parser_t *parser, const uint8_t *pound) {
             // If we didn't get an valid interpolation, then this is just regular
             // string content. This is like if we get "#@-". In this case the caller
             // should keep lexing.
-            parser->current.end = variable;
+            parser->current.end = pound + 1;
             return YP_TOKEN_NOT_PROVIDED;
         }
         case '$':