curl/parseconfig: fix mem-leak

When looping, first trying '.curlrc' and then '_curlrc', the function
would not free the first string.

Closes #4731

src/tool_parsecfg.c

@@ -97,6 +97,8 @@ int parseconfig(const char *filename, struct GlobalConfig *global)
       int i = 0;
       char prefix = '.';
       do {
+        /* if it was allocated in a previous attempt */
+        free(pathalloc);
         /* check for .curlrc then _curlrc in the home dir */
         pathalloc = curl_maprintf("%s%s%ccurlrc", home, DIR_CHAR, prefix);
         if(!pathalloc) {