Fix possible leak in String

Fixes: espressif#710

Author: me-no-dev

cores/esp32/WString.cpp

@@ -23,6 +23,7 @@
 
 #include "WString.h"
 #include "stdlib_noniso.h"
+#include "esp32-hal-log.h"
 //extern "C" {
 //#include "esp_common.h"
 //}
@@ -165,19 +166,23 @@ unsigned char String::reserve(unsigned int size)
 
 unsigned char String::changeBuffer(unsigned int maxStrLen)
 {
- size_t newSize = (maxStrLen + 16) & (~0xf);
- char *newbuffer = (char *) malloc(newSize);
+ size_t newSize = ((maxStrLen + 16) & (~0xf)) - 1;
+ char *newbuffer = (char *) realloc(buffer, newSize+1);
  if(newbuffer) {
- memset(newbuffer, 0, newSize);
- memcpy(newbuffer, buffer, len);
- if (buffer) {
- free(buffer);
+ if(newSize > len){
+ if(newSize > capacity){
+ memset(newbuffer+capacity, 0, newSize-capacity);
+ }
+ } else {
+ //new buffer can not fit the old len
+ newbuffer[newSize] = 0;
+ len = newSize;
  }
- capacity = newSize - 1;
+ capacity = newSize;
  buffer = newbuffer;
  return 1;
  }
- buffer = newbuffer;
+ log_e("realloc failed! Buffer unchanged");
  return 0;
 }