src: fix compiler warnings in node_buffer.cc

Currently the following compiler warnings are generated on Linux:
../src/node_buffer.cc:
In function 'void node::Buffer::{anonymous}::StringSlice(
    const v8::FunctionCallbackInfo<v8::Value>&)
      [with node::encoding encoding = (node::encoding)1]':
../src/node_buffer.cc:54:20: warning:
'start' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   if (end < start) end = start;
                    ^~~
../src/node_buffer.cc:50:10: note: 'start' was declared here
   size_t start;
          ^~~~~

This commit initializes start and end to zero to avoid these warnings.

PR-URL: https://github.com/nodejs/node/pull/25665


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>