test543: extended to verify zero length input

As was reported in #5601

tests/data/test543

@@ -30,6 +30,8 @@ curl_easy_escape
 <verify>
 <stdout>
 %9C%26K%3DI%04%A1%01%E0%D8%7C%20%B7%EFS%29%FA%1DW%E1
+IN: '' OUT: ''
+IN: ' 12' OUT: '%2012'
 </stdout>
 </verify>
 </testcase>

tests/libtest/lib543.c

@@ -49,11 +49,21 @@ int test(char *URL)
 
   s = curl_easy_escape(easy, (const char *)a, asize);
 
-  if(s)
+  if(s) {
     printf("%s\n", s);
-
-  if(s)
     curl_free(s);
+  }
+
+  s = curl_easy_escape(easy, "", 0);
+  if(s) {
+    printf("IN: '' OUT: '%s'\n", s);
+    curl_free(s);
+  }
+  s = curl_easy_escape(easy, " 123", 3);
+  if(s) {
+    printf("IN: ' 12' OUT: '%s'\n", s);
+    curl_free(s);
+  }
 
   curl_easy_cleanup(easy);
   curl_global_cleanup();