Fixed bug in 'string.format' with option '%f'

As an example, 'print(string.format("%.99f", 1e70))' may have a
lot of garbage after the number.

The old test to ensure that 'string.format("%.99f", n)' was not too
large, 'fabs(n) < 1e100', assumes that the number will fit in the 99
bytes; but the 99 is not the space for the number, it is the added
extra zeros.  The option worked for smaller numbers because of the
extra space added to MAX_ITEM.

lstrlib.c

@@ -1227,16 +1227,14 @@ static int str_format (lua_State *L) {
           nb = lua_number2strx(L, buff, maxitem, form,
                                   luaL_checknumber(L, arg));
           break;
-        case 'e': case 'E': case 'f':
-        case 'g': case 'G': {
+        case 'f':
+          maxitem = MAX_ITEMF;  /* extra space for '%f' */
+          buff = luaL_prepbuffsize(&b, maxitem);
+          /* FALLTHROUGH */
+        case 'e': case 'E': case 'g': case 'G': {
           lua_Number n = luaL_checknumber(L, arg);
-          if (*(strfrmt - 1) == 'f' && l_mathop(fabs)(n) >= 1e100) {
-            /* 'n' needs more than 99 digits */
-            maxitem = MAX_ITEMF;  /* extra space for '%f' */
-            buff = luaL_prepbuffsize(&b, maxitem);
-          }
           addlenmod(form, LUA_NUMBER_FRMLEN);
-          nb = l_sprintf(buff, maxitem, form, (LUAI_UACNUMBER)n);
+          nb = snprintf(buff, maxitem, form, (LUAI_UACNUMBER)n);
           break;
         }
         case 'p': {

testes/strings.lua

@@ -255,6 +255,12 @@ do    -- longest number that can be formatted
   local s = string.format('%.99f', -(10^i))
   assert(string.len(s) >= i + 101)
   assert(tonumber(s) == -(10^i))
+
+  -- limit for floats
+  assert(10^38 < math.huge)
+  local s = string.format('%.99f', -(10^38))
+  assert(string.len(s) >= 38 + 101)
+  assert(tonumber(s) == -(10^38))
 end