Fix stack size detection on Linux (fix #1427)

ChangeLog

@@ -1,6 +1,7 @@
             Increase jslMatch error buffer size to handle "UNFINISHED TEMPLATE LITERAL" string (#1426)
             nRF5x: Make FlashWrite cope with flash writes > 4k
             Increase max size of native strings on platforms that support it - 16 bit to 32 bit (#1432)
+            Fix stack size detection on Linux (fix #1427)
 
      1v98 : Allow Crypto SHA1 without SHA256/512 (for ESP8266 where flash is scarce)
             Add better docs for the form of Wifi callback functions

src/jsutils.c

@@ -844,7 +844,9 @@ size_t jsuGetFreeStack() {
   char ptr; // this is on the stack
   extern void *STACK_BASE;
   uint32_t count =  (uint32_t)((size_t)STACK_BASE - (size_t)&ptr);
-  return 1000000 - count; // give it 1 megabyte of stack
+  const uint32_t max_stack = 1000000; // give it 1 megabyte of stack
+  if (count>max_stack) return 0;
+  return max_stack - count;
 #else
   // stack depth seems pretty platform-specific :( Default to a value that disables it
   return 1000000; // no stack depth check on this platform