vm: fix compile warnings on linux

In function ‘readlink’,
    inlined from ‘vm_executable_path’ at vm/os-linux.cpp:13:27:
/usr/include/x86_64-linux-gnu/bits/unistd.h:68:10: warning: ‘__readlink_alias’ specified size 18446744073709551614 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
   68 |   return __glibc_fortify (readlink, __len, sizeof (char),
      |          ^
/usr/include/x86_64-linux-gnu/bits/unistd-decl.h: In function ‘vm_executable_path’:
/usr/include/x86_64-linux-gnu/bits/unistd-decl.h:71:16: note: in a call to function ‘__readlink_alias’ declared with attribute ‘access (write_only, 2, 3)’
   71 | extern ssize_t __REDIRECT_NTH (__readlink_alias,
      |                ^

vm/os-linux.cpp

@@ -3,18 +3,18 @@
 namespace factor {
 
 const char* vm_executable_path() {
-  ssize_t bufsiz = 4096;
+  size_t bufsiz = 4096;
 
   // readlink is called in a loop with increasing buffer sizes in case
   // someone tries to run Factor from a incredibly deeply nested
   // path.
   while (true) {
     char* buf = new char[bufsiz + 1];
-    ssize_t size= readlink("/proc/self/exe", buf, bufsiz);
+    ssize_t size = readlink("/proc/self/exe", buf, bufsiz);
     if (size < 0) {
       fatal_error("Cannot read /proc/self/exe", errno);
     } else {
-      if (size < bufsiz) {
+      if (size < ((size_t) bufsiz)) {
         // Buffer was large enough, return string.
         buf[size] = '\0';
         const char* ret = safe_strdup(buf);