Fix bugs

#1814411 "Incorrect SQL when changing structure on a field"
#1814293 "Incorrect SQL when changing structure on a FLOAT field"

source/helpers.pas

@@ -288,9 +288,14 @@ var
   p1,p2        : Integer;
 begin
   p1 := pos('(', str);
-  if p1 = 0 then p2 := Length(str) + 1
-  else for p2:=strlen(pchar(str)) downto 0 do if str[p2] = ')' then break;
-  result := copy (str, p1+1, p2-p1-1);
+  Result := '';
+  // Only return something if opening bracket was found, otherwise empty string
+  if p1 > 0 then
+  begin
+    for p2:=strlen(pchar(str)) downto 0 do
+      if str[p2] = ')' then break;
+    result := copy (str, p1+1, p2-p1-1);
+  end;
 end;