Fix #2612 - struct mg_str::ptr -> buf

src/str.h

@@ -2,14 +2,12 @@
 
 #include "arch.h"
 
+// Describes an arbitrary chunk of memory
 struct mg_str {
-  const char *ptr;  // Pointer to string data
-  size_t len;       // String len
+  char *buf;   // String data
+  size_t len;  // String length
 };
 
-#define MG_C_STR(a) \
-  { (a), sizeof(a) - 1 }
-
 // Using macro to avoid shadowing C++ struct constructor, see #1298
 #define mg_str(s) mg_str_s(s)