GDALGridCreate with NULL progress callback function

[yilmaz]

In the information header of function "GDALGridCreate" (and therefore in the corresponding documentation), it is stated that the progress callback function and its associated argument can be passed as NULL:

@param pfnProgress a GDALProgressFunc() compatible callback function for
reporting progress or NULL.
@param pProgressArg argument to be passed to pfnProgress.  May be NULL.

In the function body, however, the callback function is called regardless of its being NULL, which leads to memory access violation.

[rouault]

r17849 /trunk/gdal/alg/gdalgrid.cpp: Fix segfault when GDALGridCreate() is called with pfnProgress == NULL (#3189) r17850 /branches/1.6/gdal/alg/gdalgrid.cpp: Fix segfault when GDALGridCreate() is called with pfnProgress == NULL (#3189)