Fix out of bound access in cgi module (#11578)

When an HTTP request with a zero CONTENT_LENGTH is made,
attempting to access addr(result[0]) raise an exception as the 0 index
is out of bound

(cherry picked from commit 181350f6c9)

lib/pure/cgi.nim

@@ -77,6 +77,8 @@ proc getEncodedData(allowedMethods: set[RequestMethod]): string =
     if methodPost notin allowedMethods:
       cgiError("'REQUEST_METHOD' 'POST' is not supported")
     var L = parseInt(getEnv("CONTENT_LENGTH").string)
+    if L == 0:
+      return ""
     result = newString(L)
     if readBuffer(stdin, addr(result[0]), L) != L:
       cgiError("cannot read from stdin")