qimage/producer_kdenlivetitle.c: add error handling

qimage/producer_kdenlivetitle.c: add error handling

Fixes Coverity CID 709363: Argument cannot be negative (NEGATIVE_RETURNS)
Function "ftell(f)" returns a negative number.
Assigning: signed variable "lSize" = "ftell".
 37                lSize = ftell (f);
 38                rewind (f);
 39
 40                char *infile = (char*) mlt_pool_alloc(lSize);
"lSize" is passed to a parameter that cannot be negative.

and compiler warning:

producer_kdenlivetitle.c: In function ‘read_xml’:
producer_kdenlivetitle.c:49:19: warning: ‘infile’ may be used uninitialized in this function [-Wmaybe-uninitialized]