linux-user: Tidy loader_exec

Reorg the if cases to reduce indentation.
Test for 4 bytes in the file before checking the signatures.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/linuxload.c

@@ -154,31 +154,31 @@ int loader_exec(int fdexec, const char *filename, char **argv, char **envp,
 
     retval = prepare_binprm(bprm);
 
-    if (retval >= 0) {
-        if (bprm->buf[0] == 0x7f
-                && bprm->buf[1] == 'E'
-                && bprm->buf[2] == 'L'
-                && bprm->buf[3] == 'F') {
-            retval = load_elf_binary(bprm, infop);
+    if (retval < 4) {
+        return -ENOEXEC;
+    }
+    if (bprm->buf[0] == 0x7f
+        && bprm->buf[1] == 'E'
+        && bprm->buf[2] == 'L'
+        && bprm->buf[3] == 'F') {
+        retval = load_elf_binary(bprm, infop);
 #if defined(TARGET_HAS_BFLT)
-        } else if (bprm->buf[0] == 'b'
-                && bprm->buf[1] == 'F'
-                && bprm->buf[2] == 'L'
-                && bprm->buf[3] == 'T') {
-            retval = load_flt_binary(bprm, infop);
+    } else if (bprm->buf[0] == 'b'
+               && bprm->buf[1] == 'F'
+               && bprm->buf[2] == 'L'
+               && bprm->buf[3] == 'T') {
+        retval = load_flt_binary(bprm, infop);
 #endif
-        } else {
-            return -ENOEXEC;
-        }
+    } else {
+        return -ENOEXEC;
     }
-
-    if (retval >= 0) {
-        /* success.  Initialize important registers */
-        do_init_thread(regs, infop);
+    if (retval < 0) {
         return retval;
     }
 
-    return retval;
+    /* Success.  Initialize important registers. */
+    do_init_thread(regs, infop);
+    return 0;
 }
 
 bool imgsrc_read(void *dst, off_t offset, size_t len,