Fix integer overflow vulnerability in exportImagePixels

[vah13]

Fixed a critical integer overflow in Imagick::exportImagePixels() that could lead to heap buffer overflow. The vulnerability occurred when calculating pixel buffer size (map_len * width * height) using 32-bit signed integers, which would overflow for large images and result in undersized buffer allocation followed by out-of-bounds memory writes.

Changes:

• Changed map_size type from int to size_t to handle large values
• Added overflow checks before calculating map_size
• Added overflow check before memory allocation
• Prevents potential heap corruption and RCE exploits

The fix validates calculations at each step:

1. Check map_len * width for overflow
2. Check result * height for overflow
3. Check final size * sizeof(type) for overflow