Fix bug in const String& AsyncWebServerRequest::arg(const __FlashStringHelper * data) const (me-no-dev#334)

Fixes crash in AsyncWebServerRequest::arg with flash string param (eg: `request->arg(F("<arg name>"))` crashes). Solution is to call `strcpy_P` instead of `strcpy`, since the variable `p` is of type `PGM_P` and not `char *`.

Author: CarlosRDomin

src/WebRequest.cpp

@@ -894,7 +894,7 @@ const String& AsyncWebServerRequest::arg(const __FlashStringHelper * data) const
   size_t n = strlen_P(p);
   char * name = (char*) malloc(n+1);
   if (name) {
-    strcpy(name, p);   
+    strcpy_P(name, p);
     const String & result = arg( String(name) ); 
     free(name); 
     return result;