Avoid unnecessary ldexp() and pow() calls

Parse times for some large .obj files (without asan):

         File A  File B  File C
Before   1239ms   294ms   271ms
After    1037ms   203ms   190ms

tiny_obj_loader.h

@@ -472,8 +472,8 @@ static bool tryParseDouble(const char *s, const char *s_end, double *result) {
   }
 
 assemble:
-  *result =
+  *result = (sign == '+' ? 1 : -1) *
-      (sign == '+' ? 1 : -1) * ldexp(mantissa * pow(5.0, exponent), exponent);
+    (exponent ? ldexp(mantissa * pow(5.0, exponent), exponent) : mantissa);
   return true;
 fail:
   return false;