Dereference null return value in gdfx.c

[Former user]

Issue : gdImageStringFTCircle() may possibly return NULL pointer.

im3 = gdImageSquareToCircle (im2, radius);

im3 can hold NULL value when 0 returned from gdImageSquareToCircle .

No Check for this implented in code.

[Pierre Joye]

fix #113, NULL deref in gdfx

→ <<cset 81d341c8f17b>>