Prevent ICE when calling parse_attribute without an attribute

Fixes 5729

`parse_attribute` will panic if the first token is not a `#`. To prevent
this we return early instead of trying to parse an invalid attribute.

src/parse/macros/cfg_if.rs

@@ -34,6 +34,11 @@ fn parse_cfg_if_inner<'a>(
             if !parser.eat_keyword(kw::If) {
                 return Err("Expected `if`");
             }
+
+            if !matches!(parser.token.kind, TokenKind::Pound) {
+                return Err("Failed to parse attributes");
+            }
+
             // Inner attributes are not actually syntactically permitted here, but we don't
             // care about inner vs outer attributes in this position. Our purpose with this
             // special case parsing of cfg_if macros is to ensure we can correctly resolve