fix memory leak in asm/nasm.c

[13579and2468]

compile it with asan

git clone https://github.com/netwide-assembler/nasm.git
cd nasm
sh autogen.sh;
AFL_USE_ASAN=1 ./configure CC=afl-gcc CXX=afl-g++ LD=afl-gcc--disable-shared;
AFL_USE_ASAN=1 make;

run

$ ./nasm myinput/test1  
myinput/test1: warning: default output file same as input, using `nasm.out' for output
 [-w+other]
nasm: fatal: unable to open input file `myinput/test1' No such file or directory

=================================================================
==14512==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 14 byte(s) in 1 object(s) allocated from:
    #0 0x7f10ccd7a808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x559a339ff044 in nasm_malloc nasmlib/alloc.c:55

SUMMARY: AddressSanitizer: 14 byte(s) leaked in 1 allocation(s).

Analysis :

When the first argument of nasm is a pathname without '.', forget to free outname before set outname = "nasm.out";.

[cyrillos]

Looks reasonable, but please make the proper changelog, see example here https://nasm.us/patches.php