BUILD: sample: avoid build warning in sample.c

BUILD: sample: avoid build warning in sample.c

Recent commit 9631a28 ("MEDIUM: sample: Extend functionality for field/word
converters") introduced this minor build warning that this patch addresses :

 src/sample.c: In function 'sample_conv_word':
 src/sample.c:2108:8: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]
 src/sample.c:2137:8: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]

No backport is needed.