fix: Correct access out of bonds on buffer used by flash functions

This commit fixes access out of bonds from flash.cpp.
With this fix it is possible to use the flash functions (write and info)
with source compile with gcc >= 11

mrfCommon/src/flash.cpp

@@ -371,14 +371,16 @@ void CFIFlash::busyWait(double timeout, unsigned n)
 CFIStreamBuf::CFIStreamBuf(CFIFlash& flash)
     :flash(flash)
     ,pos(0u)
-{}
+{
+    buf.resize(1);
+}
 
 CFIStreamBuf::int_type CFIStreamBuf::underflow()
 {
     // read-ahead is only one page
-    buf.resize(flash.pageSize());
+    buf.resize(std::max(1u, flash.pageSize()));
     flash.read(pos, buf.size(), (epicsUInt8*)&buf[0]);
-    setg(&buf[0], &buf[0], &buf[buf.size()]);
+    setg(&buf[0], &buf[0], &buf[buf.size()-1u]);
     pos += buf.size();
 
     return buf[0];