tjLoadImage(): Fix FPE triggered by malformed BMP

In rdbmp.c, it is necessary to guard against 32-bit overflow/wraparound
when allocating the row buffer, because since BMP files have 32-bit
width and height fields, the value of biWidth can be up to 4294967295.
Specifically, if biWidth is 1073741824 and cinfo->input_components = 4,
then the samplesperrow argument in alloc_sarray() would wrap around to
0, and a division by zero error would occur at line 458 in jmemmgr.c.

If biWidth is set to a higher value, then samplesperrow would wrap
around to a small number, which would likely cause a buffer overflow
(this has not been tested or verified.)