Fix std/base64.decode out of bounds read (#23526)

inputLen may end up as 0 in the loop if the input string only includes
trailing characters. e.g. without the patch, decode(" ") would panic.

(cherry picked from commit 30cf570af9)

lib/pure/base64.nim

@@ -239,7 +239,7 @@ proc decode*(s: string): string =
     inputLen = s.len
     inputEnds = 0
   # strip trailing characters
-  while s[inputLen - 1] in {'\n', '\r', ' ', '='}:
+  while inputLen > 0 and s[inputLen - 1] in {'\n', '\r', ' ', '='}:
     dec inputLen
   # hot loop: read 4 characters at at time
   inputEnds = inputLen - 4

tests/stdlib/tbase64.nim

@@ -17,6 +17,8 @@ template main() =
   doAssert encode("") == ""
   doAssert decode("") == ""
 
+  doAssert decode(" ") == ""
+
   const testInputExpandsTo76 = "+++++++++++++++++++++++++++++++++++++++++++++++++++++++++"
   const testInputExpands = "++++++++++++++++++++++++++++++"
   const longText = """Man is distinguished, not only by his reason, but by this