fixes more nil string bugs

lib/packages/docutils/rstgen.nim

@@ -312,7 +312,6 @@ proc setIndexTerm*(d: var RstGenerator, id, term: string,
   ## The index won't be written to disk unless you call `writeIndexFile()
   ## <#writeIndexFile>`_. The purpose of the index is documented in the `docgen
   ## tools guide <docgen.html#index-switch>`_.
-  assert(not d.theIndex.isNil)
   var
     entry = term
     isTitle = false

lib/pure/strutils.nim

@@ -1366,7 +1366,9 @@ proc find*(s: string, sub: char, start: Natural = 0, last: Natural = 0): int {.n
       if sub == s[i]: return i
   else:
     when hasCStringBuiltin:
-      let found = c_memchr(s[start].unsafeAddr, sub, last-start+1)
+      let L = last-start+1
+      if L > 0:
+        let found = c_memchr(s[start].unsafeAddr, sub, L)
         if not found.isNil:
           return cast[ByteAddress](found) -% cast[ByteAddress](s.cstring)
     else:
@@ -1515,7 +1517,7 @@ proc replace*(s, sub: string, by = ""): string {.noSideEffect,
   elif subLen == 1:
     # when the pattern is a single char, we use a faster
     # char-based search that doesn't need a skip table:
-    var c = sub[0]
+    let c = sub[0]
     let last = s.high
     var i = 0
     while true:

lib/system/sysstr.nim

@@ -110,6 +110,7 @@ proc nimToCStringConv(s: NimString): cstring {.compilerProc, inline.} =
   else: result = cstring(addr s.data)
 
 proc copyStr(s: NimString, start: int): NimString {.compilerProc.} =
+  if s == nil: return nil
   result = copyStrLast(s, start, s.len-1)
 
 proc toNimStr(str: cstring, len: int): NimString {.compilerProc.} =