Fixed bug #75139 (libgd/gd_interpolation.c:1786: suspicious if ?)

We back-port https://github.com/libgd/libgd/commit/dd48286 even though
we cannot come up with a regression test, because the erroneous
condition appears to be impossible to trigger.

We also parenthesize the inner ternary operation to avoid confusion.

NEWS

@@ -10,6 +10,7 @@ PHP                                                                        NEWS
 
 - GD:
   . Fixed bug #75124 (gdImageGrayScale() may produce colors). (cmb)
+  . Fixed bug #75139 (libgd/gd_interpolation.c:1786: suspicious if ?). (cmb)
 
 - Intl:
   . Fixed bug #75090 (IntlGregorianCalendar doesn't have constants from parent

ext/gd/libgd/gd_interpolation.c

@@ -1783,8 +1783,8 @@ gdImagePtr gdImageRotateGeneric(gdImagePtr src, const float degrees, const int b
 
 	const gdFixed f_slop_y = f_sin;
 	const gdFixed f_slop_x = f_cos;
-	const gdFixed f_slop = f_slop_x > 0 && f_slop_x > 0 ?
-							f_slop_x > f_slop_y ? gd_divfx(f_slop_y, f_slop_x) : gd_divfx(f_slop_x, f_slop_y)
+	const gdFixed f_slop = f_slop_x > 0 && f_slop_y > 0 ?
+							(f_slop_x > f_slop_y ? gd_divfx(f_slop_y, f_slop_x) : gd_divfx(f_slop_x, f_slop_y))
 						: 0;