FindCURL: Fix list index check after search

Fix logic added by commit fc5afbe970 (FindCURL: support COMPONENTS to
check features, 2018-11-28, v3.14.0-rc1~287^2~2).

When searching for particular components and `curl-config` reports one of
the components being searched for first in the list, `find_package` fails.
This is due to the check that treats non-zero index in the list as success
and zero index as failure, while documentation on `list(FIND)` states that
failure to find an element results in return value of -1 (not 0). I'm
hitting this when building cURL with support for HTTP and HTTPS protocols
only, and then trying to `find_package(CURL COMPONENTS HTTP HTTPS)`.

I'm using `if(NOT x EQUAL -1)` check form as it appears to be the most used
throughout the modules.

While fixing this issue I've looked through all the uses of `list(FIND)` in
other modules but wasn't able to find improper use except here.

Modules/FindCURL.cmake

@@ -148,16 +148,16 @@ if(CURL_FIND_COMPONENTS)
   endif()
   foreach(component IN LISTS CURL_FIND_COMPONENTS)
     list(FIND CURL_KNOWN_PROTOCOLS ${component} _found)
-    if(_found)
+    if(NOT _found EQUAL -1)
       list(FIND CURL_SUPPORTED_PROTOCOLS ${component} _found)
-      if(_found)
+      if(NOT _found EQUAL -1)
         set(CURL_${component}_FOUND TRUE)
       elseif(CURL_FIND_REQUIRED)
         message(FATAL_ERROR "CURL: Required protocol ${component} is not found")
       endif()
     else()
       list(FIND CURL_SUPPORTED_FEATURES ${component} _found)
-      if(_found)
+      if(NOT _found EQUAL -1)
         set(CURL_${component}_FOUND TRUE)
       elseif(CURL_FIND_REQUIRED)
         message(FATAL_ERROR "CURL: Required feature ${component} is not found")