elf_get_section_* should return 0 if section wasn't found

Otherwise it won't be possible to differentiate between an error and
not-found condition.  It seems some callers (MIPS, PPC backends) already
count on that behavior anyway.
diff --git a/ltrace-elf.c b/ltrace-elf.c
index 7de9155..aff585e 100644
--- a/ltrace-elf.c
+++ b/ltrace-elf.c
@@ -141,8 +141,9 @@
 			return 0;
 		}
 	}
-	return -1;
 
+	*tgt_sec = NULL;
+	return 0;
 }
 
 static int