Fix a few malloc() and free() calls

Replace them with jsonp_malloc() and jsonp_free() to support the
custom memory allocation.

src/hashtable.c

@@ -126,7 +126,7 @@ static int hashtable_do_del(hashtable_t *hashtable,
     if(hashtable->free_value)
         hashtable->free_value(pair->value);
 
-    free(pair);
+    jsonp_free(pair);
     hashtable->size--;
 
     return 0;

src/strbuffer.c

@@ -19,7 +19,7 @@ int strbuffer_init(strbuffer_t *strbuff)
     strbuff->size = STRBUFFER_MIN_SIZE;
     strbuff->length = 0;
 
-    strbuff->value = malloc(strbuff->size);
+    strbuff->value = jsonp_malloc(strbuff->size);
     if(!strbuff->value)
         return -1;
 
@@ -30,7 +30,7 @@ int strbuffer_init(strbuffer_t *strbuff)
 
 void strbuffer_close(strbuffer_t *strbuff)
 {
-    free(strbuff->value);
+    jsonp_free(strbuff->value);
     strbuff->size = 0;
     strbuff->length = 0;
     strbuff->value = NULL;