fix: do not try to assemble file named by $JWASM / %JWASM% variable

[ecm-pushbx]

If the variable named JWASM was set, jwasm (both DOS32/DJGPP and Linux) would parse the contents of that variable as a command line parameter. For example,

edrdos/repo/drbio$ JWASM=FOO jwasm -c -Zm -Fo./bin/initmsgs initmsgs.asm
JWasm v2.18, Aug 14 2023, Masm-compatible assembler.
Portions Copyright (c) 1992-2002 Sybase, Inc. All Rights Reserved.
Source code is available under the Sybase Open Watcom Public License.

Fatal error A1106: Cannot open file: "FOO" [ENOENT]
initmsgs.asm: 93 lines, 2 passes, 0 ms, 0 warnings, 0 errors

This code is responsible for filling argv[0]:

JWasm/src/main.c

Line 92 in dbeeaf6

pEnv = getenv( "JWASM" );

The error became apparent because the assembler spammed a lot of messages when trying to replace the use of MASM in the EDR-DOS build by JWasm. They were caused by setting set JWASM=C:\BIN\JWASM.EXE and JWasm trying to parse its executable file as an assembly source file then.

[ecm-pushbx]

Oh, I just noticed that this variable is probably intended to allow passing parameters like NASM's NASMENV. In that case it is intended to parse the contents as switches and the fault lies with me.