Fix bugs in search::context().

- Don't add context if words == 0
- Handle beginning / end of text correctly.

src/search.cpp

@@ -153,27 +153,54 @@ match_context
 context(const boost::match_results<std::string::const_iterator> &match,
         std::uint64_t words)
 {
+    if (words == 0)
+    {
+        return {};
+    }
+
     const auto &prefix{match.prefix().str()};
     const auto &suffix{match.suffix().str()};
     size_t pos_before{prefix.length()};
     size_t pos_after{};
+
     ++words;
 
     while (words != 0)
     {
-        if (pos_before != std::string::npos)
+        if (pos_before != 0)
         {
-            pos_before = prefix.rfind(' ', pos_before) - 1;
+            pos_before = prefix.rfind(' ', pos_before);
+            if (pos_before != std::string::npos)
+            {
+                --pos_before;
+            }
+            else
+            {
+                pos_before = 0;
+            }
         }
 
         if (pos_after != std::string::npos)
         {
-            pos_after = suffix.find(' ', pos_after) + 1;
+            pos_after = suffix.find(' ', pos_after);
+            if (pos_after != std::string::npos)
+            {
+                ++pos_after;
+            }
         }
         words -= 1;
     }
 
-    return {prefix.substr(pos_before + 2), suffix.substr(0, pos_after - 1)};
+    if (pos_before != 0)
+    {
+        pos_before += 2;
+    }
+    if (pos_after != std::string::npos)
+    {
+        pos_after -= 1;
+    }
+
+    return {prefix.substr(pos_before), suffix.substr(0, pos_after)};
 }
 
 std::string headline(const std::string_view prefix)