fix(linux): fix off-by-one buffer overflow in process_parse_stat (#2324)

When slash_pos is 0 (executable path starts with '/'), the null
terminator was written to cmdline_procname[BUFFER_LEN], which is one
byte past the end of the array.

Subtract 1 from both the strncpy length and the null terminator index
so they are always within the [0, BUFFER_LEN-1] bounds of the buffer.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: paddlesteamer

src/data/os/linux.cc

@@ -3026,8 +3026,9 @@ static void process_parse_stat(struct process *process) {
     strncpy(cmdline_procname, cmdline, BUFFER_LEN);
   } else {
     long int slash_pos = slash_ptr - tmpstr;
-    strncpy(cmdline_procname, cmdline + slash_pos + 1, BUFFER_LEN - slash_pos);
-    cmdline_procname[BUFFER_LEN - slash_pos] = 0;
+    strncpy(cmdline_procname, cmdline + slash_pos + 1,
+            BUFFER_LEN - slash_pos - 1);
+    cmdline_procname[BUFFER_LEN - slash_pos - 1] = 0;
   }
 
   /* Extract cpu times from data in /proc filesystem */