trurl: fix misbehavior on empty query param

Add several tests to verify.

Reported-by: Peter Schilling
Fixes #351

Author: bagder

tests.json

@@ -2783,5 +2783,78 @@
             "stderr": "",
             "returncode": 0
         }
+    },
+    {
+        "input": {
+            "arguments": [
+                "http://example.org/?a=&b=1"
+            ]
+        },
+        "expected": {
+            "stdout": "http://example.org/?a=&b=1\n",
+            "stderr": "",
+            "returncode": 0
+        }
+    },
+    {
+        "input": {
+            "arguments": [
+                "http://example.org/?a=1&b="
+            ]
+        },
+        "expected": {
+            "stdout": "http://example.org/?a=1&b=\n",
+            "stderr": "",
+            "returncode": 0
+        }
+    },
+    {
+        "input": {
+            "arguments": [
+                "http://example.org/?a=1&b=&c=2"
+            ]
+        },
+        "expected": {
+            "stdout": "http://example.org/?a=1&b=&c=2\n",
+            "stderr": "",
+            "returncode": 0
+        }
+    },
+    {
+        "input": {
+            "arguments": [
+                "http://example.org/?a=1&b=&c=2",
+                "--json"
+            ]
+        },
+        "expected": {
+            "stdout":[
+                {
+                    "url": "http://example.org/?a=1&b=&c=2",
+                    "parts": {
+                        "scheme": "http",
+                        "host": "example.org",
+                        "path": "/",
+                        "query": "a=1&b=&c=2"
+                    },
+                    "params": [
+                        {
+                            "key": "a",
+                            "value": "1"
+                        },
+                        {
+                            "key": "b",
+                            "value": ""
+                        },
+                        {
+                            "key": "c",
+                            "value": "2"
+                        }
+                    ]
+                }
+            ],
+            "stderr": "",
+            "returncode": 0
+        }
     }
 ]

trurl.c

@@ -1378,24 +1378,38 @@ static struct string *memdupzero(char *source, size_t len, bool *modified)
     else {
       int llen;
       int rlen;
+      int rightside;
 
       /* decode both sides */
       left = decodequery(source, (int)(sep - source), &llen);
       if(!left)
         goto error;
-      right = decodequery(sep + 1,
-                          (int)len - (int)(sep - source) - 1, &rlen);
-      if(!right)
-        goto error;
+
+      /* length on the right side of '=': */
+      rightside = (int)len - (int)(sep - source) - 1;
+
+      if(rightside) {
+        right = decodequery(sep + 1,
+                            (int)len - (int)(sep - source) - 1, &rlen);
+        if(!right)
+          goto error;
+      }
+      else {
+        right = NULL;
+        rlen = 0;
+      }
+
       /* encode both sides again */
       el = encodequery(left, llen);
       if(!el)
         goto error;
-      er = encodequery(right, rlen);
-      if(!er)
-        goto error;
+      if(right) {
+        er = encodequery(right, rlen);
+        if(!er)
+          goto error;
+      }
 
-      encode = curl_maprintf("%s=%s", el, er);
+      encode = curl_maprintf("%s=%s", el, er ? er : "");
       if(!encode)
         goto error;
     }