Use vec.data() instead of &vec[0]

In general `vec.data()` is always safe, whereas `&vec[0]`
may fail when `vec` is empty.

src/link/output.cpp

@@ -174,7 +174,7 @@ static void writeBank(std::deque<struct Section const *> *bankSections, uint16_t
 			}
 
 			// Output the section itself
-			fwrite(&section->data[0], 1, section->size, outputFile);
+			fwrite(section->data.data(), 1, section->size, outputFile);
 			if (overlayFile) {
 				// Skip bytes even with pipes
 				for (uint16_t i = 0; i < section->size; i++)