fix memory leak in eval_strfunc

When running with -fsanitize=leak enabled nasm prints this error:

==19965==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 360 byte(s) in 90 object(s) allocated from:
    #0 0x7faee9396867 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    netwide-assembler#1 0x5645d39a401c in nasm_malloc nasmlib/alloc.c:55
    netwide-assembler#2 0x5645d3a41f9d in string_transform asm/strfunc.c:356
    netwide-assembler#3 0x5645d3a37d40 in eval_strfunc asm/eval.c:761
    netwide-assembler#4 0x5645d3a37d40 in expr6 asm/eval.c:906
    netwide-assembler#5 0x5645d3a3968d in expr5 asm/eval.c:627
    netwide-assembler#6 0x5645d3a39aca in expr4 asm/eval.c:602
    netwide-assembler#7 0x5645d3a39b72 in expr3 asm/eval.c:563
    netwide-assembler#8 0x5645d3a39db8 in expr2 asm/eval.c:537
    netwide-assembler#9 0x5645d3a39f38 in expr1 asm/eval.c:511
    netwide-assembler#10 0x5645d3a3a0b8 in expr0 asm/eval.c:485
    netwide-assembler#11 0x5645d3a3a242 in rexp3 asm/eval.c:422
    netwide-assembler#12 0x5645d3a3a508 in rexp2 asm/eval.c:396
    netwide-assembler#13 0x5645d3a3a6a8 in rexp1 asm/eval.c:369
    netwide-assembler#14 0x5645d3a3a838 in rexp0 asm/eval.c:342
    netwide-assembler#15 0x5645d3a3a838 in cexpr asm/eval.c:305
    netwide-assembler#16 0x5645d3a3ad08 in bexpr asm/eval.c:298
    netwide-assembler#17 0x5645d3a3ad08 in evaluate asm/eval.c:1032
    netwide-assembler#18 0x5645d39e4f20 in parse_line asm/parser.c:959
    netwide-assembler#19 0x5645d399e243 in assemble_file asm/nasm.c:1735
    netwide-assembler#20 0x5645d3998801 in main asm/nasm.c:719
    netwide-assembler#21 0x7faee8aaed8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    netwide-assembler#22 0x7faee8aaee3f in __libc_start_main_impl ../csu/libc-start.c:392
    netwide-assembler#23 0x5645d399acd4 in _start (/home/ivan/d/nasm/nasm+0x2e5cd4)

SUMMARY: AddressSanitizer: 360 byte(s) leaked in 90 allocation(s).

This problem is reproducible on test utf.asm.

The problem was caused by the fact that eval_strfunc doesn't free the string
allocated by string_transform.

asm/eval.c

@@ -766,6 +766,7 @@ static expr *eval_strfunc(enum strfunc type, const char *name)
     }
 
     val = readstrnum(string, string_len, &rn_warn);
+    nasm_free(string);
     if (parens) {
         scan();
         if (tt != ')') {