boot, oemboot: fix to abort properly on file not found

Prior to this commit the first cluster specified from the last
compared directory entry would be used if the file is not found.
Fix the wrong (boot.asm) or missing (oemboot.asm) jump to the
error handler so that on file not found an error is displayed.

Reference: https://sourceforge.net/p/freedos/bugs/97/
"Bugs in FAT12/16 boot sector loaders if file not found"
This commit fixes the most crucial bug of this 2012-09-27 report.

boot/boot.asm

@@ -266,7 +266,7 @@ next_entry:     mov     cx, 11
                 cmp     byte [es:di], 0 ; if the first byte of the name is 0,
                 jnz     next_entry      ; there is no more files in the directory
 
-                jc      boot_error      ; fail if not found
+                jmp     boot_error      ; fail if not found
 ffDone:
                 push    ax              ; store first cluster number
 

boot/oemboot.asm

@@ -353,6 +353,7 @@ next_entry:     mov     cx, 11
                 jc      boot_error      ; fail if not found and si wraps
                 cmp     byte [si], 0    ; if the first byte of the name is 0,
                 jnz     next_entry      ; there are no more files in the directory
+                jmp     boot_error
 
 ffDone:
                 mov [first_cluster], ax ; store first cluster number