eina: remove logically dead code.

The for loop above would exit with at least length = 0, so < 0 is
just impossible.

CID 1039943.

src/lib/eina/eina_convert.c

@@ -271,9 +271,6 @@ eina_convert_atod(const char *src, int length, long long *m, long *e)
         exponent += *str - '0';
      }
 
-   if (length < 0)
-     goto on_length_error;
-
    if (sign < 0)
      exponent = -exponent;