set empty path

fix boostorg#426

include/boost/url/detail/any_path_iter.hpp

@@ -173,7 +173,8 @@ class enc_segs_iter
         : it_(first)
         , end_(last)
     {
-        front = *first;
+        if (it_ != end_)
+            front = *first;
     }
 
     bool
@@ -241,7 +242,8 @@ class plain_segs_iter
         : it_(first)
         , end_(last)
     {
-        front = *first;
+        if (first != last)
+            front = *first;
     }
 
     bool

include/boost/url/impl/url_base.ipp

@@ -1079,21 +1079,25 @@ edit_segments(
     // adjust capacity
     reserve(size() + n - n0);
 
-    // start of output
-    auto dest = s_ + p0;
+    if (s_)
+    {
+        // start of output
+        auto dest = s_ + p0;
 
-    // move and size
-    std::memmove(
-        dest + n,
-        s_ + p1,
-        size() - p1);
-    u_.set_size(
-        id_path,
-        u_.len(id_path) -
-            (n0 - n));
-    u_.nseg_ = nseg1;
-    s_[size()] = '\0';
-    return dest;
+        // move and size
+        std::memmove(
+            dest + n,
+            s_ + p1,
+            size() - p1);
+        u_.set_size(
+            id_path,
+            u_.len(id_path) -
+                (n0 - n));
+        u_.nseg_ = nseg1;
+        s_[size()] = '\0';
+        return dest;
+    }
+    return s_;
 }
 
 // insert or replace [i0, i1)
@@ -1148,7 +1152,7 @@ edit_segments(
         2 = "./"
         3 = "/./"
 
-    This is malleable prefix that might need to
+    This is a malleable prefix that might need to
     change according the URL scheme and authority.
 
 */
@@ -1193,8 +1197,10 @@ edit_segments(
                 ':') != string_view::npos ||
             it0.front.empty()))
     {
-        BOOST_ASSERT(nseg > 0);
-        prefix = 2;
+        if (nseg > 0)
+            prefix = 2;
+        else
+            prefix = 0;
     }
     else if(
         abs &&
@@ -1268,7 +1274,7 @@ edit_segments(
     n += prefix + suffix;
     auto dest = edit_segments(
         i0, i1, n, nseg);
-    auto const last = dest + n;
+    char const *const last = dest + n;
 
 /*  Write all characters in the destination:
 

test/unit/url.cpp

@@ -1425,6 +1425,10 @@ class url_test
                 u.set_path(pct_encoded_view(s));
                 BOOST_TEST_EQ(u.string(), match);
             };
+            check(
+                "",
+                "",
+                "");
             check(
                 "",
                 "path/to/file.txt",