fixes a critical strutils bug [backport]

(cherry picked from commit 68ce92d4eb)

lib/pure/strutils.nim

@@ -435,7 +435,7 @@ proc isNilOrWhitespace*(s: string): bool {.noSideEffect, procvar, rtl, extern: "
 proc substrEq(s: string, pos: int, substr: string): bool =
   var i = 0
   var length = substr.len
-  while i < length and s[pos+i] == substr[i]:
+  while i < length and pos+i < s.len and s[pos+i] == substr[i]:
     inc i
   return i == length