Escape dots in regex for using startupString in "code"

Otherwise, we could theoretically load a file named something like
"startupxm2xin", call "code" on something from that file, and
end up with some code from startup.m2.in.

M2/Macaulay2/m2/code.m2

@@ -57,7 +57,7 @@ code FilePosition := x -> (
      (
 	  wp := set characters " \t\r);";
 	  file := (
-	       if match("startup.m2.in$", filename) then startupString
+	       if match("startup\\.m2\\.in$", filename) then startupString
 	       else if filename === "currentString" then (
 		    if currentString === null
 		    then error "code no longer available"