Fix out-of-bound read for invalid XML

In the given example, there is an invalid field:
To: sut <sip:[service]@[remote_ip]:"remote_port]>

There are no more quotes later. When we search for the terminating
quote, nothing is found, so we skip to the end of the string. Then
the loop continues, we have p++ and continue beyond the buffer.

Fixes #727.

src/message.cpp

@@ -130,7 +130,9 @@ static char* quoted_strchr(const char* s, int c)
     for (p = s; *p && *p != c; p++) {
         if (*p == '"') {
             p++;
-            p += strcspn(p, "\"");
+            p += strcspn(p, "\"\n");
+            if (!*p)
+                break;
         }
     }