Jared Lundell filed bug report #1604956

(http://curl.haxx.se/bug/view.cgi?id=1604956) which identified setting
CURLOPT_MAXCONNECTS to zero caused libcurl to SIGSEGV. Starting now, libcurl
will always internally use no less than 1 entry in the connection cache.

lib/url.c

@@ -377,11 +377,13 @@ CURLcode Curl_close(struct SessionHandle *data)
 }
 
 /* create a connection cache of a private or multi type */
-struct conncache *Curl_mk_connc(int type)
+struct conncache *Curl_mk_connc(int type,
+                                int amount) /* set -1 to use default */
 {
   /* It is subject for debate how many default connections to have for a multi
      connection cache... */
-  int default_amount = (type == CONNCACHE_PRIVATE)?5:10;
+  int default_amount = amount == -1?
+    ((type == CONNCACHE_PRIVATE)?5:10):amount;
   struct conncache *c;
 
   c= calloc(sizeof(struct conncache), 1);
@@ -407,6 +409,20 @@ CURLcode Curl_ch_connc(struct SessionHandle *data,
   long i;
   struct connectdata **newptr;
 
+  if(newamount < 1)
+    newamount = 1; /* we better have at least one entry */
+
+  if(!c) {
+    /* we get a NULL pointer passed in as connection cache, which means that
+       there is no cache created for this SessionHandle just yet, we create a
+       brand new with the requested size.
+    */
+    data->state.connc = Curl_mk_connc(CONNCACHE_PRIVATE, newamount);
+    if(!data->state.connc)
+      return CURLE_OUT_OF_MEMORY;
+    return CURLE_OK;
+  }
+
   if(newamount < c->num) {
     /* Since this number is *decreased* from the existing number, we must
        close the possibly open connections that live on the indexes that